311    073 


THE  ROBERT   E.  COWAN  COLl. 

I'RKSKNTICI)    TO    Till. 

UNIVERSITY  OF  CALIFORNIA 


C.  P.  HUNTINGTON 

dUNE.   1897. 

Recession  H.Q...^       Class  No, 


ROASTING 


OF 


GOLD  AND  SILVER  ORES 


AND   THE 


EXTRACTION 


OF   THEIR 


RESPECTIVE   METALS 

WITHOUT  QUICKSILVER, 


BY    Cr.    liUSTEL, 

MINING   ENGINEER    AND   METALLURGIST, 

Author  of  "  Nevada  and  California  Processes  of  Silver  and  Gold  Extraction, 
and  "  Concentration  of  all  Kinds  of  Ores." 


Illustrated  with  Numerous   Engravings. 


PUBLISHED  AND  SOLD  BY 

DEWEY  &  CO.,   PROPRIETORS   SCIENTIFIC    PRESS. 

SAN   FRANCISCO,  1870. 


Entered  according  to  Act  of  Congress,  in  the  year  1870, 

BY  G.  KUSTEL, 

In  the  Clerk's  Office  of  the  District  Court  of  the  United  States  for 
the  Northern  District  of  California. 


Printed  by  SPAULDING  &  BARTO, 

SCIENTIFIC  PRESS  BOOK  AND  JOB  OFFICE, 

414  Clay  Street,  San  Francisco. 


PREFACE. 


The  publication  of  this  Treatise  is  due  solely  to  the  many 
inquiries  concerning  the  "Leaching,  Solving  and  Precipita- 
tion Process  for  Silver  Ores, ' '  now  successfully  practiced  in 
Sonora,  Mexico,  where  it  has  been  lately  introduced  by  Mr. 
Ottocar  Hofmann. 

In  consideration  of  the  very  important  preparation  of  the 
ore,  before  it  is  subjected  to  the  Solving  Process, — namely, 
the  Roasting, — I  have  thought  it  proper  to  devote  consider- 
able space  to  the  description  of  different  modifications  of 
this  operation,  which  is  regulated  by  the  peculiarity  of  the 
ore,  and  by  the  subsequent  treatment.  It  is  impossible  to 
give  any  one  way  which  will  be  suitable  in  every  case  ;  for 
this  reason,  and  in  order  to  cover  all  cases  as  far  as  possible, 
a  detailed  description  of  different  modes  of  Boasting  will  not 
be  superfluous. 

The  Solving  Process,  as  now  practiced,  is  a  very  econom- 
ical method  for  the  extraction  of  silver,  for  the  reason  that  no 
quicksilver  and  no  castings  are  used  except  what  are  needed 
for  crushing.  Mills  in  Mexico  being  dependent  on  San  Fran- 
cisco for  the  shoes,  dies,  gearing,  etc.,  of  amalgamating  pans, 
millmen  there  know  how  to  appreciate  a  process  confined  to 
wooden  tubs  requiring  no  power.  A  comparatively  small 
capital  is  necessary  for  building  up  such  works,  and  hence 
there  is  a  more  reasonable  ratio  established  between  the 
amount  of  money  which  must  be  expended  on  the  works  and 
the  real  value  of  the  mine,  than  where  other  more  expensive 


IV  PREFACE. 

machinery  is  employed, — a  circumstance  which,  being  insuf- 
ficiently regarded,  is  often  the  source  of  failure. 

Mr.  O.  Hofmann  commenced  first  with  the  "  Chlorination 
Process"  (§82),  but  finding  great  difficulty  in  obtaining  the 
regular  supply  of  sulphuric  acid  and  manganese  from  San 
Francisco,  abandoned  the  chlorination  with  cold  chlorine  gas, 
which  is  indispensable  in  the  presence  of  gold.  Another  dif- 
ficulty was  in  obtaining  a  good  article  of  sulphide  of  sodium. 
He  tried  to  extract  the  potash  from  ashes,  and  to  use  this  in 
place  of  soda,  but  decided  finally  in  favor  of  lime,  which  is 
found  in  abundance.  From  this  the  sulphide  of  calcium  is 
easily  manufactured  on  the  spot.  Sulphide  of  calcium  was 
first  applied  by  Kiss  ($80). 

The  Solving  Process  is  very  simple,  and  readily  performed 
by  common  wrorkmen  ;  besides  the  lime,  only  brimstone 
must  be  provided,  in  order  to  prepare  the  necessary  chem- 
icals for  solving  and  precipitation.  It  is  a  general  but 
erroneous  belief,  that  the  solving  is  a  slow  process.  An 
amalgamating  pan  is  charged  with  500  to  1,000  pounds  of 
roasted  ore,  and  treated  at  least  six  hours,  and  therefore 
turns  out  at  most  two  tons  in  24  hours  ;  while  a  box  or  vat 
of  proper  size  used  in  the  Solving  Process,  can  work  from 
four  to  five  tons  in  the  same  time. 

Only  those  ores  are  treated  by  this  process  which  abso- 
lutely require  roasting  ;  which,  however,  with  improved  fur- 
naces, is  not  so  expensive  as  it  used  to  be.  The  chloride 
ores  alone  can  be  leached  directly  without  roasting,  and  this 
when  there  is  no  other  silver  combination  in  them. 

G.  KUSTEL. 

MABCH,  1870. 


I.    INTRODUCTION. 


Classification  of  Ores. 

1.  Ores  ma}7    be  classified  :    «.   According  to 
the  metal,  the  extraction  of  which  is  principally 
remunerative  ;     as   silver  ores,  lead  ores,    copper 
ores,  etc.     b.  According  to  the  metallurgical  treat- 
ment ;  as  roasting  ores,  smelting  ores,  amalgamat- 
ing ores,    etc.     c.  According  to   the  predominant 
gangue,  as    calcareous   ores,  quartzose  or  ochery 
ores.     d.  According  to  the  predominant  metallic 
mineral;  as  sulphuret  ores,  chloride  ores,  carbon- 
ates, etc. 

Important  Silver  Ores. 

2.  The   most   important  silver  ores  are  those 
found  in  such  quantities   as   to   be   an   object  of 
metallurgical  operations.     The  principal  minerals 
of  this  kind  are  the  following  : 

A.  Real  Silver  0?*es.  a.  Sulphuret  of  Silver,  or 
silver  glance,  with  87  per  cent,  of  silver.  It  is  of 
common  occurrence,  and  is  the  most  suitable  of  the 
silver  sulphurets  for  pan  amalgamation  without 


6  INTRODUCTION. 

roasting,  b.  Brittle  Silver  Ore,  or  sulphuret  of 
silver  and  antimony.  This  mineral  contains  68  per 
cent,  of  silver,  and  is  quite  common,  c.  Polyba- 
site,  sulphuret  of  silver,  antimony  and  some  arsenic, 
with  75  per  cent,  of  silver.  Brittle  silver  ore  and 
polybasite  are  both  tractable  in  pans  without  roast- 
ing, although  not  so  readily  as  the  simple  sulphuret. 
All  other  sulphureted  silver  ores  require  roasting. 
d.  Ruby  Silver.  The  dark  red  silver  ore,  or  antimo- 
nial  variety,  with  59  per  cent.,  and  the  light  red 
silver  ore,  or  arsenical  variety,  with  65  per  cent, 
of  silver,  are  valuable  minerals.  They  occur  quite 
frequently  in  Nevada,  Idaho,  Montana,  Mexico, 
etc.  e.  Miargyrite,  sulphuret  of  silver  and  anti- 
mony; 36.5  per  cent,  of  silver;  Idaho,  Montana, 
etc.  /.  Stromeyerite,  or  silver  copper  glance,  a 
sulphuret  of  silver  and  copper  containing  up  to  53 
per  cent,  of  silver;  Nevada,  Arizona,  etc.  g. 
Horn  Silver,  or  chloride  of  silver,  with  75  per  cent, 
of  silver;  occurs  massive  in  White  Pine,  Nevada; 
prepared  by  nature  for  the  pan  amalgamation,  h. 
Stetefeldtite  and  Partzite,  with  up  to  25  per  cent,  of 
silver,  are  oxide  ores  which  occur  very  frequently 
in  Nevada,  Arizona,  etc. 

B.  Argentiferous  Ores.  a.  Silver-fahl-ore ,  ar- 
gentiferous gray  copper  ore.  It  contains  silver  in 
very  variable  proportions  up  to  31  per  cent.  This 
ore  is  quite  common,  and  for  this  reason  is  im- 
portant. It  is  also  one  of  the  most  rebellious  ores, 
containing  copper,  antimony,  arsenic,  sulphur, 


INTRODUCTION.  I 

lead,  iron,  zinc,  and  sometimes  gold  and  quick- 
silver, b.  Argentiferous  Lead  Ores,  galena,  or  sul- 
phuret  of  lead,  lead  glance.  Generally,  this  is  not 
rich  in  silver,  containing  from  $20  to  $60  per  ton. 
Specimens  assay  sometimes  as  high  as  $300.  The 
fine  grained  variety  is  generally  considered  richer 
than  the  coarse  crystallized  kind,  but  this  has  not 
been  observed  to  be  the  case  in  Nevada  and  Ari- 
zona, c.  Cerusite,  carbonate  of  lead.  If  pure, 
without  admixture  of  copper  and  other  carbonates, 
it  is  poor  in  silver  in  most  cases.  Raw,  it  amalga- 
mates only  too  readily  in  pans.  Smelting  is  the 
only  proper  way  of  treating  galena  and  cerusite. 
d.  Argentiferous  Zincblende.  Sulphuret  of  zinc. 
Pure  zincblende  contains  usually  only  traces  of 
silver;  often,  however,  it  assays  well,  even  up  to 
$400  per  ton,  although  no  other  silver  ore  can  be 
detected  with  it.  In  some  mines  the  argentiferous 
zincblende  prevails,  and  is  the  most  important  ore. 
It  requires  a  great  heat  in  roasting,  e.  Argentifer- 
ous Pyrites.  Copper  and  iron  pyrites  are  poor  in 
silver,  but  often  auriferous.  Pyrite  is  a  valuable 
companion  for  silver  ores  which  have  to  be  treated 
by  a  chloridizing  roasting,  on  account  of  its  amount 
of  sulphur,  which  is  necessary  for  the  decom- 
position of  salt. 

Difference  between  Real  Silver  Ores  and 
Argentiferous  Ores. 

3.     Heal  silver  ores  have  mostly  an  unvariable 
amount  of  silver.     Heal  silver  minerals  admit  an 


8  INTRODUCTION. 

approximate  estimate  of  the  value  of  the  ore,  if  the 
proportion  of  ore  and  gangue  is  considered,  with- 
out making  an  assay.  With  the  argentiferous  ores 
it  is  different.  Fahl  ore,  for  instance,  may  be  very 
poor  or  very  rich, 'and  its  value  can  be  ascertained 
only  by  an  assay.  There  are  no  means  of  esti- 
mating the  richness  of  argentiferous  ores  "by 
sight." 

Important  Combinations. 

4.  With  the  exception  of  a  few  metal  oxides  of 
iron,  zinc,  tin,  manganese,  and,  among  silver  ores, 
of  the  stetefeldtite,  etc.,  the  most  important,  be- 
cause most  frequent  ores,  are  the  sulphureted  vari- 
eties. Sulphur  is  the  most  formidable  obstacle  to 
the  metallurgist  in  extracting  metals  from  their  re- 
spective ores.  Desulphurization  has  been  a  sub- 
ject of  most  diligent  and  numerous  experiments. 
The  oldest  method  is  the  application  of  heat,  which 
is  still  in  use,  notwithstanding  the  many  attempts 
in  modern  times  to  dispense  entirely  with  fire  or  to 
modify  its  application  so  as  to  perform  the  process 
more  perfectly  and  in  a  shorter  time. 

Means  of  Desulphurization. 

6.  The  desulphurization  of  ores  is  effected:  a. 
By  heating  with  free  admission  of  air.  This  is  the 
common  way  of  "  roasting,"  and  the  most  impor- 
tant, and  is  effected  either  in  kilns,  heaps,  etc.,  or 


INTRODUCTION.  U 

in  reverberatory  furnaces.  As  soon  as  the  sulphu- 
rated ore  is  heated  to  a  certain  degree,  one  part  of 
the  sulphur  escapes  as  sulphurous  acid;  another  is 
converted  into  sulphuric  acid,  which  is  also  decom- 
posed by  an  increased  heat.  Some  sulphurets 
(iron  pyrites)  lose  their  sulphur  without  the  appli- 
cation of  heat,  being  decomposed  by  exposure  to 
the  action  of  air  for  a  long  time.  This  way  is 
sometimes  practiced  on  gold-bearing  pyrites,  b. 
By  heating  with  exclusion  of  air.  Only  the  sul- 
phides of  gold  and  platinum  are  decomposed  per- 
fectly by  this  method.  Other  sulphureted  ores 
lose  their  sulphur  only  in  part,  being  reduced  to  a 
lower  state  of  sulphide.  Sulphuret  of  silver  (Ag 
S)  remains  undecomposed.  Cinnabar,  sulphide  of 
antimony  (Sb  S3)  and  sulphide  of  arsenic  volatilize 
unchanged.  Iron  pyrites  (Fe  S'2)  gives  up  23  per 
cent,  of  its  sulphur,  being  reduced  to  magnetic 
pyrites,  and,  by  a  strong  heat,  to  proto-sulphide  of 
iron  (Fe  S),  not  further  reducible.  Also  sulphide 
of  zinc  (zincblende),  remains  undecomposed. 
Copper  glance  retains  its  sulphur,  and  copper  py- 
rites loses  only  one  part  of  the  sulphur  which 
is  combined  with  the  iron  in  it.  Galena  (Pb  S)  is 
reduced  to  a  lower  state  (Pb4  S),  a  part  of  the  lead 
separating  out  in  a  metallic  state,  c.  By  super- 
heated steam.  Sulphurets  not  evolving  sulphur  by 
the  last  process,  lose  their  sulphur  slowly  on  the 
application  of  steam,  sulphureted  hydrogen  and 
sulj  hurous  acid  being  formed.  Experiments  made 
by  Begnault  showed  that  desulphurization  is 
1* 


10  INTRODUCTION. 

effected  more  perfectly  if  air  is  admitted.  Roast- 
ing in  reverberatory  furnaces  is  always  effected  "by 
the  oxygen  of  the  air  and  by  steam,  as  there  is  no 
fuel  used  which  contains  less  than  25  to  30  per 
cent,  of  water.  Superheated  steam  has  been  tried 
in  different  ways  on  sulphurets  with  the  highest 
expectations,  but  with  no  better  results  for  practical 
use  than  are  given  in  the  ordinary  way  by  the 
steam  obtained  from  fuel.  It  may  be  useful  in 
many  instances  to  have  more  steam  than  is  thus 
obtained,  but  this  increases  considerably  the  ex- 
pense of  roasting;  as,  for  instance,  in  Patera's  ap- 
plication of  steam  in  roasting  silver  ores,  tried 
principally  with  the  intention  of  expelling  anti- 
mony, arsenic,  etc.  Another  application  of  super- 
heated steam,  with  exclusion  of  air,  is  Hagan's 
method,  which  may  prove  successful  on  pyritous 
ores,  having  at  the  same  time  the  advantage  of 
being  a  very  cheap  method,  d.  By  heating  with 
metals,  alkalies  or  alkaline  earths,  for  which  the 
sulphur  has  a  greater  affinity.  The  affinity  of  sul- 
phur for  the  following  metals  decreases  in  the 
order  in  which  they  stand,  being  strongest  for  the 
first  and  weakest  for  the  last:  Copper,  iron,  tin, 
zinc,  lead,  silver,  antimony,  arsenic.  Each  of 
these  metals  can  be  desulphurized  by  the  next  pre- 
ceding, though  with  difficulty;  but  more  easily  by 
one  further  off.  Practical  use  of  this  property  is 
made  in  smelting  galena  with  the  addition  of  me- 
tallic iron  or  iron  ore.  Sulphide  of  silver  in  cru- 
cibles is  decomposed  by  stirring  the  liquid  with 


INTRODUCTION.  11 

red  hot  iron.  Quicksilver  is  obtained  from  cinna- 
bar by  heating  the  latter  with  lime,  which  takes  up 
the  sulphur,  etc.  e.  Carbon  has  no  great  affinity 
for  sulphur;  the  use  of  charcoal  for  desulphurization 
of  ores  is  therefore  an  inferior  method.  So  is  also 
the  use  of  carbonic  acid. 

Result  of  Desulphurization. 

6.  The  direct  extraction  of  metals  from  sul- 
phurets,  either  by  smelting  or  amalgamation,  is 
not  practicable.  In  smelting,  the  sulphurets  melt 
very  readily,  but  only  a  small  part,  if  any,  of  the 
metal  is  obtained,  while  the  greater  part  runs  out 
combined  with  the  sulphur  as  matt.  For  this  reason 
the  roasting  of  sulphureted  ore  for  the  purpose  of 
smelting  is  indispensable,  unless  iron  is  added. 
Such  roasting  or  burning  takes  often  many  weeks, 
or  months.  The  direct  amalgamation,  also,  of  sul- 
phurets gives  a  veiy  poor  result,  except  in  the  case 
of  silver  glance.  By  means  of  electricity,  com- 
bined with  the  chemical  action  of  sulphate  of  cop- 
per and  salt,  the  silver  and  gold  sulphurets  are  de- 
composed; but,  with  the  exception  of  the  patio 
amalgamation,  no  process  has  yet  been  publicly 
demonstrated  as  really  practical  for  the  treatment 
of  all  kinds  of  raw  sulphurets.  The  desulphuriza- 
tion is  therefore  still  a  most  important  preparation 
for  the  extraction  of  metals.  The  general  effect  of 
roasting  is  that  the  metals  are  oxidized.  Only 
gold  and  silver  are  transformed  into  a  metallic 


1 2  INTRODUCTION . 

condition;  and  of  the  silver,  moreover,  a  large  per- 
centage is  always  found  as  a  sulphate,  even  when 
the  roasting  is  well  performed.  Some  of  the  silver 
combines  as  an  oxide  with  antimony  and  silica,  if 
present.  All  the  oxides  obtained  by  desulphuriza- 
tion  must  be  again  deoxidized  in  order  to  get  them 
in  a  metallic  state. 

Means  of  Reduction  or  Deoxidation. 

7.  Heating  alone  will  reduce  the  oxides  of  the 
precious  metals  only.  Oxide  of  gold  does  not 
occur  in  nature,  neither  is  it  obtained  in  any  of  the 
metallurgical  processes.  Oxide  of  silver  is  also 
unimportant;  it  is  formed,  to  a  small  extent,  in  cu- 
pellation  (and  taken  up  by  the  litharge),  in  smelt- 
ing silver  ores  combined  with  silica,  and  in  roasting 
silver  ores  in  the  presence  of  antimony,  arsenic, 
etc. 

The  most  powerful  agent  of  reduction  is  carbon 
(charcoal,  coke,  etc.)  and  carbonic  oxide.  In  all 
smelting  in  blast  furnaces,  the  carbonic  oxide  is 
the  real  reducer.  The  burning  coal,  under  the  in- 
fluence of  the  compressed  air,  produces  carbonic 
acid,  melting  at  the  same  time  the  ore;  the  car- 
bonic acid,  passing  through  the  glowing  coal  above 
the  melting  region,  gives  up  a  part  of  its  oxygen  to 
the  coal,  and  is  reduced  thereby  to  carbonic  oxide, 
which  in  turn  takes  up  oxygen  again,  from  the 
metal  oxides,  reducing  them  to  a  metallic  state;  a 
contact  of  ore  oxides  with  carbon  is  therefore  not 


INTRODUCTION.  13 

necessary  for  the  purpose  of  reduction.  All  metals 
do  not  retain  their  oxygen  with  equal  tenacity,  but 
some  part  with  it  much  more  easily  than  others. 
For  instance,  lead,  copper,  bismuth,  antimony, 
cobalt  and  nickel,  require  for  their  reduction  a 
darker  or  lighter  red  heat,  while  iron,  zinc  and  tin 
are  reduced  only  at  a  white  heat.  But  also  hydro- 
gen and  carbureted  hydrogen,  created  by  the 
burning  fuel,  are  powerful  reducing  agents. 

Metal  oxides  in  solution  are  reduced  and  precip- 
itated in  a  metallic  condition  by  other  metals.  On 
this  principle  copper  is  precipitated  by  metallic 
iron,  which  goes  into  solution  in  place  of  the  cop- 
per; sulphate  of  silver,  in  Ziervogel's  process,  is 
precipitated  by  copper,  etc.  Also,  by  aid  of  the 
electro-galvanic  stream,  metals  are  reduced  to  a 
metallic  state  from  their  solutions. 

Desulphurization  of  Silver  Ores  not  Effi- 
cient. 

8.  Although,  by  mere  desulphurization  the 
silver  is  to  a  great  extent  converted  into  a  metallic 
state,  this  is  not  always  its  most  suitable  condition 
except  for  smelting.  Almost  all  silver  extracted  in 
the  United  States  is  obtained  by  amalgamation, 
smelting  being  confined  to  a  few  localities  where 
the  ore  contains  such  a  high  percentage  of  lead 
that  its  amalgamation  is  impossible.  It  would 
seem  as  if  metallic  silver  should  amalgamate  more 
easily  than  if  combined  with  another  substance. 


14  INTRODUCTION. 

t 

This,  however,  is  not  the  case.  The  silver,  after 
roasting,  is  generally  coated  with  the  oxides  of 
volatile  base  metals,  which  prevent  its  ready  amal- 
gamation. Moreover,  a  direct  contact  between 
quicksilver  and  silver  is  a  necessary  condition  for 
their  amalgamation.  A  momentary  contact  in  a 
muddy  pulp  is  not  always  successful.  The  chlo- 
ride of  silver,  however,  goes  into  solution  and 
unites  easily  with  the  quicksilver.  Hence,  in  most 
instances,  it  is  necessary  to  adopt  a  chloridizing 
roasting. 

What  a  Chloride  is,  and  how  Chlorination 
is  Effected. 

9.  The   term   chloride  is   applied   to  all  com- 
pounds of  chlorine  with  a  metal  or  other  radical. 
Chlorine  is  a  greenish-yellow  gas,  an  elementary 
substance,  of  2.45  specific  gravity,  and  of  a  peculiar 
and  disagreeable  odor.     It  is  not  found  free  in  na- 
ture, but  always  in  combination,  principally  with 
sodium,  forming  common  salt.     Metallic  chlorides 
are  of   frequent  occurrence.     Chlorine  is,  for  in- 
stance, combined  with  silver  as  horn  silver,  with 
copper  as  Atacamite,  with  lead  as  Kerasine,  Mendi- 
pite,  etc;  also  with  quicksilver  as  Calomel. 

10.  To  chloridize  ore, — that  is,  to  convert  the 
metals  into  chlorides, — it  is  necessary  to  produce 
chlorine  and  to  bring  it  in  intimate  contact  with 
the  ore  particles.     The  cheapest  material  evolving 


INTRODUCTION.  15 

chlorine  is  salt  (chloride  of  sodium),  and  the  only 
practical  way  of  separating  the  chlorine  from  sodi- 
um is  by  substituting  for  it  another  substance  for 
which  the  sodium  has  a  stronger  affinity.  The 
cheapest  ingredient  for  this  purpose  is  sulphuric 
acid.  The  sodium  being  oxidized  to  soda,  unites 
with  the  sulphuric  acid,  forming  sulphate  of  soda, 
while  chlorine  is  set  free. 

For  the  treatment  of  ores  there  are  two  principal 
methods  of  chloridizing.  One  is  roasting  the  ore 
with  salt  in  a  furnace;  the  other  is  the  "  cold  chlo- 
rination."  Boasting,  at  first,  when  in  the  presence 
of  salt,  has  an  oxidizing  effect,  as  there  is  then  no 
sulphuric  acid  present  to  decompose  the  salt,  and 
the  heat  alone  would,  if  increased,  volatilize  and 
not  decompose  this.  The  sulphurets  in  the  ore, 
under  the  influence  of  heat,  lose  a  part  of  their 
sulphur  as  sulphurous  acid  gas;  the  other  part  of 
the  sulphur  oxidizes  to  sulphuric  acid.  As  soon  as 
this  is  formed  it  attacks  the  salt,  and  the  chlo- 
rine, being  set  free,  then  acts  on  metals,  metal 
oxides,  sulphurets,  arseniurets  and  antimgnial  com- 
binations, forming  partly  metal  chlorides  and 
partly  chlorides  of  sulphur,  arsenic  and  antimony. 

11.  The  other  mode  of  chloridizing  consists  in 
the  employment  of  cold  chlorine  gas  with  roasted 
ores,  principally  desulphurized  gold  ores,  but  of 
late,. also  silver  ores.  The  chlorine  must  be  pro- 
duced here  separately,  and  conducted  into  the  cold 
ore  by  leaden  or  india  rubber  pipes.  The  ingredi- 
ents are :  salt,  sulphuric  acid  and  peroxide  of  man- 


16  INTRODUCTION. 

ganese.  Salt  is  first  attacked  by  the  sulphuric 
acid,  and  hydrochloric  acid  and  sulphate  of  soda 
are  formed.  The  hydrogen  of  the  hydrochloric 
acid  then  combines  with  the  oxygen  of  the  manga- 
nese, and  the  chlorine  escapes.  A  part  of  the 
chlorine  unites  with  the  manganese,  but  is  decom- 
posed again  by  sulphuric  acid,  so  that  all  chlorine 
is  expelled  from  the  salt,  leaving  sulphates  of  soda 
and  manganese  in  the  gas  generator.  The  chlori- 
nation  of  gold,  unlike  that  of  silver,  is  difficult  to 
effect  in  a  furnace  (§  38),  for  the  reason  that,  if 
formed,  the  gold  chloride  is  reduced  back  to  the 
metallic  state  at  a  low,  almost  dark  red  heat.  The 
difference  between  hot  and  cold  chlorination  is 
principally  found  in  the  fact  that,  while  in  the  first 
way  a  great  many  base  metal  chlorides  are  formed, 
the  cold  chlorine  combines  principally  with  the 
free  metal,  with  silver  and  gold;  while  the  other 
metals,  being  oxidized,  are  not  decomposed  by  the 
chlorine.  Silver  oxide,  if  present,  is  decomposed 
and  chloridized. 

Chlorination  is  also  effected  by  chemical  decom- 
position in  the  wet  way,  as  practiced  in  the  Mexi- 
can patio  amalgamation,  by  mixing  with  the  ore 
sulphate  of  copper  and  salt. 

Means  of  Separating  the  Metal  from  Chlo- 
rine. 

13.  The  chloride  of  silver  can  be  melted  with- 
out being  altered;  chlorides  of  gold  and  of  plati- 


INTRODUCTION .  1 7 

num.  lose  all  their  chlorine  on  being  heated. 
Chloride  of  iron  exposed  to  air  and  heat,  as  is  the 
case  in  a  chloridizing  roasting,  loses  its  chlorine 
and  is  changed  to  iron  oxide.  The  chloride  "of 
copper  gives  up  only  a  part  of  its  chlorine.  Heat- 
ing alone  has  therefore  no  practical  value  for  the 
disengagement  of  chlorine. 

The  most  effective  way  of  separating  the  chlorine 
from  the  metal  is  the  application  of  another  metal 
for  which  the  chlorine  has  more  affinity.  On  this 
property  of  chlorine  is  based  the  amalgamation  of 
silver  ores,  after  a  chloridizing  roasting,  in  pans, 
tubs  and  barrels,  and  the  patio  amalgamation. 
The  chloride  of  silver  in  the  ore  is  decomposed, 
and  the  silver  set  free  during  amalgamation  in  iron 
pans  by  the  metallic  iron  of  the  pan,  or  if  quick- 
silver is  charged  at  the  same  time  with  the  ore,  by 
both  the  quicksilver  and  the  iron.  In  the  barrel 
amalgamation  the  silver  is  disengaged  by  metallic 
iron,  and  in  the  patio  amalgamation  by  quicksilver. 
In  all  these  instances  the  silver,  being  deprived  of 
its  chlorine,  alloys  with  the  quicksilver  and  forms 
the  amalgam. 

On  the  same  principle  the  metal  is  extracted 
from  soluble  chlorides.  The  proto-chlorides  are 
all  more  or  less  soluble  in  water,  except  that  of  sil- 
ver, which  is  quite  insoluble.  The  chloride  of 
copper,  in  solution,  is  brought  together  with  me- 
tallic iron,  or  conveyed  over  it.  The  chlorine  of 
the  copper  unites  with  the  iron,  and  the  copper 
falls  in  a  metallic  state,  ready  to  be  melted  into 


18  INTKODUCTION. 

bars,  after  being  washed,  pressed  and  dried.  In- 
directly, the  silver  is  extracted  from  its  chloridized 
state  by  dissolving  the  chloride  in  the  hyposul- 
phites of  soda,  of  potash,  or  of  lime.  In  these  salts 
the  silver  chloride  dissolves  very  readily,  giving  a 
clear  solution  of  a  very  sweet  taste,  out  of  which 
the  silver  is  precipitated  by  the  corresponding 
alkaline  sulphides  as  sulphide  of  silver. 

The  chloride  of  gold,  obtained  from  the  chlo- 
rination  of  gold-bearing  sulphurets,  is  precipitated 
by  sulphate  of  iron  in  such  a  way  that  metallic  gold 
results,  while  the  chlorine  combines  with  a  part  of 
the  iron. 

13.  The  silver  is  easily  obtained  from  the  chlo- 
ride by  melting  it  with  alkalies;  for  instance,  with 
soda,  potash  or  lime.  The  chlorine  unites  with 
sodium,  calcium,  etc.,  and  the  silver  separates  on 
the  bottom  of  the  crucible.  If  there  is  not  a  suffi- 
cient amount  of  the  alkalies  present,  some  silver 
will  be  lost.  In  most  instances  it  is  preferable  to 
mix  the  artificial  chloride  with  water  and  some  sul- 
phuric acid  and  granulated  zinc,  or]  zinc  sheet  if 
smaller  quantities  are  being  operated  on.  The 
chloride  of  silver  by  degrees  changes  its  white., 
color  to  a  dark  gray,  being  converted  into  the  me- 
tallic state  in  a  short  time.  It  is  reduced  to 
metal  by  the  nascent  hydrogen.  After  the  sulphate 
of  zinc,  which  is  formed  and  dissolved,  has  been 
washed  away,  the  silver  is  pressed,  dried,  and,  with 
addition  of  some  soda  and  borax,  melted  into  a 
bar. 


'ROASTING  OF  ORES.  19 

In  the  same  way  as  from  a  sulphate,  silver  can 
be  precipitated  by  copper,  after  the  chloride  of 
silver  has  been  dissolved  in  a  hot  solution  of  salt, 
as  is  done  in  Augustin's  process.  This  is  not  prac- 
ticable with  the  argentiferous  solution  of  hyposul- 
phite of  soda. 

By  using  sodium  amalgam  and  iron  filings,  the 
silver  chloride  is  instantly  decomposed  and  silver 
amalgam  formed. 

The  chloride  of  gold  is  precipitated  in  a  metallic 
condition;  also  by  the  chloride  of  iron  (Fe  Cl),  the 
consideration  of  which  is  important  in  treating  sul- 
phurets  by  chlorination. 


II.    ROASTING  OF  ORES. 


14.  The  object  of  roasting  is  either  to  effect 
chemical  changes,  as  required  for  amalgamation, 
smelting,  etc.,  or  sometimes  also  to  reduce  the 
hardness  of  the  ore,  in  order  to  make  it  easier  to 
crush.  Roasting  for  the  latter  purpose,  exposing 
the  ore  to  the  fire  in  large  pieces,  is  more  properly 
termed  "  burning."  The  beginning  of  smelting  is 
under  all  circumstances  beyond  the  limits  of  roast- 
ing; therefore  all  roasting  furnaces  in  which  the 
regulation  of  heat  is  so  far  out  of  the  control  of  the 


20  BOASTING    OF    ORES.  * 

roaster  that  a  partial  smelting  would  arise,  are  un- 
fit for  roasting.  This  is  often  the  case  with  vertical 
furnaces.  But  although  a  partial  smelting  or  clot- 
ting is  not  within  the  province  of  roasting,  and  in 
all  instances  is  very  injurious  to  the  result  of  sub- 
sequent amalgamation  or  precipitation,  it  is  never- 
theless applied  with  much  success  on  concentrated 
ore  intended  for  smelting.  By  this  process  the 
loose  sand  assumes  a  compact  form,  the  gases  and 
wind  penetrate  the  charge  more  easily,  and  the  loss 
in  metal  is  diminished. 

If  there  is  no  necessity  for  effecting  a  perfect 
chemical  change  in  the  ore,  or  if  roasting  is  re- 
quired for  smelting  purposes,  and  a  powdered  form 
is  not  admissible,  the  ore  is  taken  in  larger  or 
smaller  pieces, — generally  not  below  the  size  of  a 
hen's  egg, — and  subjected  to  roasting  either  in  open 
heaps,  in  kilns  or  in  vertical  or  reverberatory  fur- 
naces. In  roasting  in  heaps,  the  wood  is  first 
placed  on  the  ground,  sometimes  surrounded  by  a 
wall  two  or  three  feet  high,  then  the  ore  is  put 
over  it.  Less  frequently  ore  and  wood  are  laid 
in  strata.  If  there  is  sufficient  sulphur  in  the  ore, 
the  burning  will  continue  without  addition  of  fuel 
for  many  days  or  weeks.  It  is  evident  that  the  re- 
sult of  such  roasting  is  very  unequal,  the  outside 
being  more  oxidized  than  the  inside,  the  heat 
greater  near  the  fuel  than  further  off,  etc.  For 
this  reason  such  ore  is  often  roasted  over  several 
times. 

In  vertical  furnaces,  the  ore  is  laid  in  strata  alter- 
nating with  fuel,  or  there  are  several  fire-places 


ROASTING    OF    ORES.  21 

outside  the  furnace  so  arranged  that  the  flame 
is  conducted  by  the  draft  into  the  furnace.  A 
modification  in  construction  and  principle  is  the 
Hagaii  roasting  furnace,  in  which  the  decom- 
position of  superheated  steam  is  a  source  of  cre- 
ating heat  and  a  decomposing  agent  at  the  same 
time.  The  roasting  is  performed  in  a  short  time, 
and  with  proper  ore  and  pieces  of  the  right  size 
the  result  is  very  satisfactory.  It  is  also  a  cheap 
process/ and  is  applied  for  roasting  gold-quartz 
holding  sulphurets,  the  amalgamation  of  which, 
without  roasting,  is  defective.  This  kind  of  roast- 
ing would  be  also  applicable  as  preparatory  for 
amalgamating  silver  ores  or  for  the  chlorination 
process  (§  74). 

In  most  instances  with  silver  ores,  a  most  perfect 
chemical  change  is  a  condition  on  which  the  result 
of  extracting  the  silver  depends  ;  and  for  this  pur- 
pose the  ore  must  be  pulverized,  in  order  to  effect 
a  perfect  contact  between  ore  particles,  gases,  and 
other  substances  which  are  mixed  with  the  ore  for 
certain  purposes.  The  roasting  of  the  pulverized 
ore  is  executed  mostly  in  reverberatory  furnaces; 
sometimes,  also,  in  a  kind  of  retort  furnace,  if  the 
roasting  should  be  done  without  the  admission  of 
air.  There  are  also  other  furnaces  lately  intro- 
duced or  tried,  the  description  of  which  will  be 
found  hereafter. 

In  accordance  with  the  intended  mode  of  ex- 
traction, the  ore  is  either  roasted  with  an  addition 
of  charcoal  powder,  whereby  the  silver  is  reduced 


22  BOASTING    OF    ORES. 

to  a  metallic  state, — a  procedure  of  little  practical 
use, — or  the  ore  is  subjected  to  an  oxidizing  roast- 
ing, with  the  principal  object  of  driving  out 
arsenic,  antimony  or  sulphur,  converting  at  the 
same  time  the  silver  into  a  sulphate  (Ziervogel's 
process);  or  a  chloridizing  roasting  is  effected,  that 
is,  roasting  with  salt. 

A.    Chloridizing  Roasting. 

^ 

15.  In  order  to  chloridize  the  ore,  an  addition 
of  common  salt  is  indispensable.  The  salt  furnishes 
chlorine  for  that  purpose,  and  is  decomposed  by 
sulphuric  acid.  The  sulphuric  acid  is  created  by 
the  decomposition  of  sulphurets  present  in  the 
ore  (§10).  .It  follows  that  if  silver  ore  is  to  be 
roasted  successfully  with  salt,  there  must  be  a  cer- 
tain percentage  of  sulphurets  in  it;  otherwise  no 
sulphuric  acid  can  be  obtained,  and  consequently 
no  chlorination,  or  at  least  only  a  very  imperfect 
one,  can  be  effected  (§  18). 

Before  introducing  the  ore  into  the  furnace  the 
latter  must  be  gradually  heated  up,  which  may  take 
ten  to  fifteen  hours.  When  nearly  red  hot,  a  charge 
of  dry  ore,  mixed  with  salt,  is  brought  on  the  hearth 
through  the  roof  and  spread  out  equally  by  means 
of  a  hoe.  The  fire  is  kept  up  moderately,  but  suf- 
ficient flame  must  be  seen  over  the  ore.  The  draft 
is  lessened  by  the  damper,  and  the  ore  stirred  dili- 
gently, but  not  continually.  The  intervals,  how- 
ever, must  be  short.  In  case  the  ore  contains  lead 


OP   TH3 

UNIVERSITY 


BOASTING    OF    ORES. 


and  antimony,  it  is  advisable  to  stir  continually 
for  at  least  three  hours.  The  ore  by  degrees  be- 
comes red  hot,  and  the  burning  of  the  sulphur  is 
quite  lively.  One  part  of  the  sulphur,  by  the 
action  of  oxygen,  is  converted  into  sulphuric  acid 
and  combines  with  the  metals,  deprived  of  their 
sulphur  or  arsenic,  to  a  sulphate.  The  period  of 
the  formation  of  sulphates  is  very  important  and 
requires  some  time  before  it  is  finished.  If  there 
is  a  large  amount  of  sulphurets  in  the  ore,  the 
burning  of  the  sulphur  creates  so  much  heat  that 
the  feeding  of  the  fire  must  be  stopped  almost  en- 
tirely for  an  hour  or  two,  but  must  be  resumed 
again  as  soon  as  it  is  perceived  that  the  ore  com- 
mences to  cool.  The  workman  stirs  the  ore,  with 
a  hoe  or  an  iron  rake,  back  and  forward  across 
the  hearth,  moving  it  from  the  bridge  toward  the 
flue  and  back.  The  formation  of  sulphates  still 
continues  with  disengagement  of  sulphurous  gas. 
The  ore  at  the  bridge  is  more  exposed  to  heat  than 
that  on  the  opposite  side,  and  the  roaster  is  obliged 
to  change  the  ore  by  raking  it  together  into  a  long- 
heap  extending  from  the  bridge  toward  the  flue  — 
not  in  the  middle,  but  nearer  the  working  door. 
By  means  of  a  shovel,  six  inches  by  twelve,  on  a 
long  (12-foot)  iron  handle,  the  roaster  takes  the  ore 
from  near  the  bridge  and  transfers  it  toward  the 
flue,  putting  it  behind  the  ridge  of  ore  until  he 
reaches  the  middle  of  the  furnace.  He  then  takes 
the  other  end  of  the  ridge  and  moves  it  toward 
the  bridge.  After  this  the  stirring  is  continued  in 
the  usual  way. 


24 


ROASTING    OF     ORES. 


16.  The  sulphates  react  now  on  the  salt,  and 
decompose  it  under  increased  heat,  setting  the 
chlorine  free.  A  mutual  exchange  takes  place  in 
part.  Sulphate  of  lead  changes  into  chloride  of 
lead,  which,  volatilizing  and  coming  in  contact  with 
air,  loses  one  part  of  its  chlorine  and  is  reduced  to 
a  combination  of  oxy-chloride  of  lead.  Sulphate 
of  iron  and  sulphate  of  copper  change  also  into 
chlorides.  The  copper  chloride  becomes  volatile, 
colors  the  flame  blue,  emits  chlorine  gas  and  forms 
subchloride  of  copper.  The  chlorine,  set  free,  de- 
composes the  sulphurets  and  sulphates  of  silver, 
and  creates  chloride  of  silver.  If,  during  the  op- 
eration, lumps  are  formed,  in  case  the  ore  was  not 
dry  enough  or  too  much  heat  was  applied  in  the 
beginning,  they  must  be  crushed  to  powder  by 
a  hammer-like  iron  instrument  with  a  long  handle. 
As  soon  as  the  chlorination  begins,  after  three  or 
four  hours,  a  different  smell,  that  of  chlorine,  will 
be  observed.  White  fumes  arise,  and  gases  and 
vapors  are  evolved,  consisting  of  sulphurous  acid, 
chlorine,  hydrochloric  acid  gas,  chloride  of  sulphur, 
of  iron  and  of  copper. 

The  ore  increases  now  in  volume  and  assumes  a 
wooly  condition.  Another  hour's  roasting  will 
now  finish  the  chlorination.  This  last  hour's  stir- 
ring requires  a  light  red  heat  in  order  to  destroy  as 
much  as  possible  of  the  base  metal  chlorides.  If 
there  is  a  great  percentage  of  copper  and  other 
base  metals  in  the  ore,  the  roasting  may  require 
more  time,  in  order  to  decompose  the  chlorides 


ROASTING    OF    ORES. 


25 


and  sulphates,  the  presence  of  which  consumes  too 

much  iron,  and  during  amalgamation  in  barrels, 
increases  the  heat  to  such  a  de- 
gree as  to  cause  an  injurious  di- 
vision of  the  mercury  into  small 
particles  and  scum.  The  base 
metal  chlorides  are  reduced  by 
the  iron  and  also  amalgamated. 

The  changing  of  the  cooler  por- 
tion near  the  flue  with  the  hotter 
part  at  the  bridge  must  be  re- 
peated two  or  three  times  during 
the  roasting  process.  When  fin- 
ished, after  five  or  six  hours,  the 
ore  is  drawn  out  and  discharged 
through  the  discharge-hole  in  the 
bottom.  White  fumes  and  gases 
are  still  arising. 

The  hoe,  Fig.  1,  is  made  of 
J-inch  wrought  iron,  six  inches 
high  and  eight  inches  wide.  The 
rod  or  handle  must  be  fifteen  feet 
long  at  least.  This  would  render 
the  instrument  heavy  and  tiresome 
to  handle  ;  it  is  therefore  prefera- 
FIG.  2.  ble  to  use  a  piece  of  gas-pipe, 
welding  it  together  with  the  rod 

as   represented  in  Fig.  1.     The  rake  is  generally 

of  cast  iron,  and  is  shown  in  Fig.  2. 
2 


26  ROASTING    OF    ORES. 


Necessary  Amount  of  Sulphurets. 

17.  In  times  when  the  barrel  amalgamation 
was  yet  practiced  in  Freiberg  (Saxony),  long  expe- 
rience showed  that  a  large  amount  of  iron  sulphu- 
rets  was  necessary  in  order  to  decompose  the 
amount  of  salt  required  for  the  chlorination.  One 
hundred  parts  of  the  ore  were  mixed  with  150  parts 
of  borax  glass,  100  parts  of  common  glass  and  one 
part  of  resin.  This  mixture,  melted  in  an  assay 
crucible,  gave  a  button  of  matt  (sulphide  of  iron), 
the  weight  of  which  was  from  25  to  30  per  «ent.  of 
the  original  weight  of  the  ore.  If  less  matt  was 
obtained,  the  ore  was  considered  too  poor  in  sul- 
phurets  and  more  pyrites  had  to  be  added. 

There  is  not  much  silver  ore  found  in  the  State 
of  Nevada  which  would  give  25  per  cent,  of  matt 
on  the  average;  and  as  there  is  no  pyrites  to  be 
obtained  for  this  purpose,  the  ore  must  be  roasted 
as  it  is.  When  starting  the  first  amalgamation 
works  in  Nevada,  I  found  from  six  to  eight  per 
cent,  of  sulphurets  (different  kinds)  in  the  Corn- 
stock  ore,  which,  after  roasting,  contained  88  per 
cent,  of  its  silver  converted  into  a  chloride.  The 
ore  from  the  Rising  Star  mine  (Idaho)  had  not  over 
8  or  10  per  cent,  of  sulphurets,  still  there  was  91 
per  cent,  of  chloride  of  silver  found  after  roasting. 
It  is,  however,  very  probable  that  from  silver  ores 
containing  a  great  deal  of  calc  spar  or  heavy  spar, 
a  less  satisfactory  result  might  be  obtained  by 


ROASTING    OF    ORES.  27 

chloridizing  roasting,  if  no  more  than  6  per  cent,  of 
sulphurets  should  occur  in  them.  Some  copper- 
holding  ores,  especially  if  other  base  metals  are 
present,  and  no  sulphur  (or  very  little),  will  give 
sometimes  a  good  chloridizing  roasting  without 
any  addition  of  green  vitriol  or  other  sulphur  com- 
bination. 

18.  In  treating  ores  entirely  free  from,  or  with 
a  very  small  percentage  of  sulphurets,  the  want  of 
sulphuric  acid  must  be  remedied  by  adding  another 
substance.  A  cheap  material  of  this  kind  is  found 
in  the  green  vitriol  or  copperas  (sulphate  of  iron), 
of  which  lij  to  3  per  cent,  is  added  when  8  to  10 
per  cent,  of  salt  is  used.  The  copperas  is  first 
calcined,  in  order  to  drive  out  its  water  fcf  crys- 
tallization, by  a  gentle  heat,  and  from  the  calcined 
article,  not  the  crystallized,  is  taken  the  above 
percentage.  This  sulphate  acts  then  on  the  salt 
the  same  as  if  it  were  created  in  roasting.  The 
copperas  is  also  added  to  arsenical  ores  free  from 
sulphurets.  But  the  percentage  of  green  vitriol 
to  be  added  depends  also  on  the  nature  of  the 
gangue.  If  there  is  a  great  deal  of  lime  in  the  ore 
it  takes  up  sulphuric  acid,  forming  sulphate  of 
lime,  remaining  in  this  condition  through  the  pro- 
cess of  roasting  without  being  decomposed  further. 
For  this  reason  calcareous  ore  requires  as  much 
more  green  vitriol  or  iron  pyrites  as  is  necessary  to 
transform  all  lime  into  a  sulphate.  Silica  or 
quartz,  if  abundant,  in  the  presence  of  steam,  de- 


28  ROASTING    OF    ORES. 

composes  some  of  the  salt  when  red  hot,  forming 
silicate  of  soda  and  hydrochloric  acid,  the  impor- 
tance of  which  is  shown  by  the  fact  that  gaseous 
hydrochloric  acid,  in  contact  with  metallic  silver, 
unites  with  it  to  a  chloride.  It  behaves  in  a  like 
manner  with  sulphur ets  and  arsenides,  of  which  the 
most  are  decomposed,  forming  chlorides,  while 
sulphur  and  arsenic  escape  combined  with  hy- 
drogen. 

Amount  of  Salt  to  be  Used,  and  When. 

19.  Ores  containing  from  80  to  100  ounces  of 
silver  per  ton  should  be  mixed  with  10  per  cent,  of 
salt.  This  is  about  the  quantity  considered  neces- 
sary iiPthe  amalgamation  works  of  Europe.  Rich 
ore  is  often  roasted  with  20  per  cent,  of  salt.  If 
all  the  chlorine  of  the  salt  could  be  transferred  to 
the  silver,  an  insignificant  amount  of  salt  only 
would  be  required  for  ores  containing  100  ounces 
of  silver — not  more  than  3J  pounds  to  the  ton;  but 
in  consequence  of  the  different  ways  in  which 
the  chlorine  decomposes  and  unites  with  base 
metals  and  gases,  the  escape  of  chlorine  from  the 
surface  of  the  ore  without  coming  in  contact  with 
the  silver,  etc.,  a  great  deal  more  of  the  salt  must 
be  applied. 

The  usual  amount  of  salt  used  in  the  United 
States  for  ores  of  the  above  value,  is  from  120  to 
140  pounds  per  ton  of  ore;  that  is,  from  6  to  7  per 
cent.  It  is  not  advisable  to  take  less  than  6  per 


ROASTING    OF    ORES.  29 

cent.  (§  37),  even  if  the  ore  be  poorer.  There  are 
instances,  however,  where  1)1  per  cent,  of  silver 
has  been  obtained  by  amalgamation  from  ores 
which  were  roasted  with  only  5  per  cent,  of  salt. 
There  was  no  natural  chloride  of  silver  in  the  ore 
when  treated  with  5  per  cent.  (Rising  Star  ore). 

As  the  salt  is  not  at  all  decomposed  before  the 
formation  of  sulphates  commences,  or  only  to  a 
very  small  extent,  it  is  also  in  this  respect  immate- 
rial whether  the  salt  is  charged  at  once  with  the 
ore,  or  whether  it  is  introduced  two  hours  later, 
unless  the  ore  is  of  such  a  nature  as  would  bake 
easily  on  a  little  increase  of  heat.  In  other 
cases,  however,  it  is  obvious  that,  taking  only  6 
per  cent,  of  salt,  and  employing  only  one  man  at  a 
furnace,  a  perfect  mixing  in  a  short  time,  as  ought 
to  be  done  if  the  salt  is  charged  after  the  sulphur 
is  burned  off,  cannot  be  expected,  and  conse- 
quently a  defective  result  will  follow.  It  is  there- 
fore under  such  circumstances  important  to  have 
the  salt  and  ore  introduced  at  the  same  time.  The 
most  perfect  application  of  the  salt  is  undoubtedly 
when  both  ore  and  salt  are  crushed  together  in  the 
battery. 

But  a  point  of  great  importance  is  the  time  when 
the  salt  should  be  added,  if  other  objects  are  in 
view.  If  the  salt  is  added  together  with  the  ore, 
or  after  the  sulphur  is  expelled  and  sulphates  are 
formed,  in  every  instance  the  base  metals  will  take 
up  their  share  of  the  chlorine,  and  therefore  more 
salt  will  be  required.  But  as  the  most  of  the 


30  ROASTING    OF    OEES. 

chlorides  are  volatile,  the  salt  is  the  means  of  get- 
ting rid  of  a  great  deal  of  the  metals  during  the 
roasting,  which  in  some  instances  is  not  very  de- 
sirable. For  instance,  if  a  great  deal  of  antimony 
and  copper  is  in  the  ore,  more  or  less  chloride  of 
silver  will  escape;  sometimes,  however,  only  a 
small  percentage.  Treating  the  ore  with  salt 
from  the  beginning,  or  adding  it  two  hours  after 
the  beginning,  the  result  is  the  same. 

A  different  result  is  obtained  if  the  salt  is  added 
after  all  the  base  metals  are  desulphurized  and  ox- 
idized. Some  base  metals,  as  antimony  and 
arsenic,  will  be  volatilized  and  thus  gotten  rid  of, 
but  not  in  so  large  a  proportion  as  if  chloridized. 
Iron  and  copper  remain  entirely  in  the  ore,  while 
both  are  volatile  as  chlorides.  The  roasting  must 
be  continued  at  a  light  red  heat  till  all  sulphates 
are  decomposed  and  the  metals  oxidized.  Apply- 
ing the  salt  after  the  dead-roasting,  the  effect  dif- 
fers from  the  above  so  far,  that  the  base  metal 
oxides  now  are  not  chloridized,  or  only  to  a  small 
extent,  while  the  silver  alone  (some  of  which  appears 
to  be  changed  to  a  metallic  state,  the  most,  how- 
ever, remaining  as  a  sulphate)  will  be  chloridized. 
But  in  order  to  effect  this  chlorination,  from  1  to  2 
per  cent,  of  green  vitriol  must  be  added  in  order 
to  accomplish  the  decomposition  of  all  the  salt. 
The  copper  is  lost  with  the  tailings  unless  smelted, 
or  extracted  by  diluted  sulphuric  acid. 


ROASTING    OF     ORES.  31 


Permanent  Stirring  not  Essential. 

20.  In  roasting  the  ore  with  salt,  a  continual 
stirring  to  the  end  of  the  process  is  not  a  necessary 
condition  for  obtaining  a  good  result.  This  de- 
pends partly  on  the  time  and  partly  on  the  nature 
of  the  ore.  As  long  as  the  ore  is  not  uniformly 
heated,  a  diligent  stirring  is  important.  The  ore 
in  the  corners  is  too  often  neglected  while  the  sul- 
phur is  burning,  and  the  exposure  of  a  fresh  sur- 
face to  the  oxygen  of  the  air  requires  also  constant 
work;  but  as" soon  as  the  smell  of  the  chlorine  is 
perceptible,  the  stirring  can  be  carried  on  at  inter- 
vals of  from  eight  to  ten  minutes.  The  chlorine 
which  is  evolved  in  the  mass  (§  23)  has  better  oppor- 
tunity to  act  on  the  metals  than  if  constantly  stirred, 
whereby  more  chlorine  escapes  up  the  chimney 
without  producing  any  effect.  This  was  proved  by 
a  comparison  of  the  work  of  two  furnaces.  A  re- 
volving furnace  had  a  speed  great  enough  to  let 
the  ore  drop  constantly  through  the  flame  and  air, 
while  the  common  furnace  was  managed  by  only 
one  man,  and  stirred  at  intervals.  Mr.  Atwood 
found  15  per  cent,  less  chloride  of  silver  in  the 
roasted  ore  from  the  revolving  furnace.  The  blame 
is  not  with  the  revolving  furnace,  but  with  the 
speed.  It  proves,  however,  that,  being  constantly 
exposed  to  the  air,  the  chlorine  escapes  with  less 
effect  than  in  the  common  furnace,  where  the  ore  is 
allowed  to  rest  for  ten  or  fifteen  minutes,  and  the 


ovol\oo!  »  hlorim-,  boin^1  in  rontai-i  \\  ith  tin1  part  • 
\vhilo    )•  'M-omyh    th<>    mass,    is    ponuit  t  o»l    lo 

form  oomhinatious.      O:llara's  imvhanioal  fun: 

Oiirh  t!u>  otv  is  romparatixolv   but  littlo  stinvil. 

6  -M  to  IM  poroont.  of  ohloritlo  of  siUor.  V 
miximv  of  i>ro,  sa\N»biNt  atul  ^all,  fornunl  into 
bricks  and  oulriiu'tl.  ^^ho^\*^l  ;!u>  silver  as  a  rhlorulo 
thfouv«>l>  tlu»  \vliolo  mass,  \vlu»rt\  as  a  niatioi-  of 
<-ou»-s(>(  (lu>  insulo  olio!  iu>t  riMiu1  into  ilinvt  cotitart 
\vitl»  air.  (\>nstant  slu>v»'liu^  is  tu^n^sarv  \\iili 
;  a  natmv.  as  it  \\*Mil,l  bako  if 


Signs  of  a  good  Chloridizing  Boasting. 


31.     A  J>XHH!  chloritli/.iiijr  roasting  should 

'.H>  JHM-  rojit.  t>f   tlu«  silvor  ronvrrtoil  intt>  i-hlo- 

;l\(M-%  aiul  sli  -  possible  of  l>ftse 

mHal  fl»lori»l««s.     To  asiMM'tnin  tlu^  amount  of 

rivlt*  of  silvor  at  tl>o  riul  of  tho  roasting,  it  is 

uwko  two  Vbv»ut   oiuMuiuro  aiul  a- 

half  is  talvtMi  tuit  i>f  tlu^  t'untavv  anvl  alKn\<\l  to 
i-oi»l.  T\\v»  0110  halt'  oinu-o  a^  u\l  out, 

ami  ono  ^No.  H  pvjvuvil  for  tho  tire  assay  rts  usual. 
Tho  *»lhor  half  oinu  :utiXHhu-v 

t'ullv  into  a  small  tiltor  in  a  j^lass  fvuuiol.  Tho  til- 
torin?:-  papor  must  pn^joot  about  ono  im-h  alx>Y6 
tlu^  QM  V  solution  of  livposulphi'r  U  is 

thon    pourod  ovor  tho  oro  in  tho  tiltor,  ami  th 
oo\\tiniu\l   as  lon^r  as  a    procipita  :iud  on 

'  :tiim  of  sulphiolo  of  sodium  to  tho  til- 


riNu   OF  OIIIM.  Il.'l 

lrrod    li-|iinl       Thin    is    Ix'.sl,   tried    inn.   rlejin    -h  • 
l.iilic.        I  f,  :i.|'l.er  lillf  i  in;;  «ir    liM.i-liin ;;  ,     I  In-    :i<  l«  1 1 1  i<  >n 
of  Hlllpllii  I"    «  »!'    '<<  xlilllll    if)    I  lie     lr.|,r||     dor,;     no|      pro 
dlicr     ;i,     preripil;|,(.c,     or    only     :i     \rry    ;,||"lil      olir,     ;  ;o 
I  h.i!     I  li«-     liiplid     !!,:;  iilllir  1    Mill  v   :i     III  Mr   d:n  I-.IT  color 
wilJioiil,  IOMIII"    il",   ped'eel,    I  r;ui;ip;irenr\  ,    Ilir  ;i  ,  ;i\ 

I       lr;i,c||('d    \\illl    U.'irill   \V,'d-e|-  ,'i.lid    |, he    Idler   1,'iKeiioTr, 
|)lll     Jlllo    ,'l     porrr|;||||      (||;.||    o|      I  I  !•.  r     \r    ,:,r|,    ;|lnl     dllcil, 

l>\    :i|)|>l\  in",   lu-.'ii  \\illi  ;in  nJcoliol   |;i,ni|).       Tin-  lillrr 
I.MI     lie     rnnovril     :iinl     liiimril     ,<i,|io\(>     Ilir     :,.!in|.|r 
\Vlirn   dr\  ,    I  1 1-     .1   :lir  ;  o|'    I  lir    lillrr    ;ilid     I  I  ir    :  :.i  1 1 1  pi  r 
Jtl'C    HllX(ul    like   Ilir   ollirr   linll     olllirr,    ;IIK|    l>o|!i    <•(  n 
ribloM  pliiccd   in  Ilir  :i;.:;;i\    lui  n;icr. 

I  I      I    ,      ,ll|l|H,     ,    ,|     !   I,    ,1      I   I,,       ,•',, ,||   |Mll       c,|       |   |n         ,,,:,, I,    ,  |       ,,,   ,          I 

uinl.  rl:il.''ii  \vli'>n   it    r.  ll,.,ip:lil    llml    il,      clilorillil.tidll   i  I  licitl  !\ 

lililhll.Ml    .      olllr|AVJH(<      I- Ml      IIIIK'll      Hlllplllll.       '.I      ;    ,U,    ,      UMilM      I,',, 
l.il    .    II     I'lll     II    <    lllc.i  li|<>.         SIlMllM    l!     IH       K   clllll.il      l<>     .!.<.)  I    illl    I  II. 

111 iiil   .  'I   pin.    chlni  [(in  M|      il  >.  i    I. ii  in.  (1,  nl,  mi;    i  n."    'l'ii  in;; 

Ilir     I. M. Illl".    II. M.       llM!l;i.tVll,    "I      lull    illl    Illllll!)     IMU'll,    Hlldlllll     l»«l 

in.-iih'.        I  ..i    Ilii  .   pin  |,.,  ...   |,,i|!   nn   ,,ini'  r  i  .   '•  .  i"li.  .1   .nil    Im    I  h. 
ir. mil  lirr  n:.  .;i\  .        \IP,I|I.  i     n.'il     ..in,,,     inn:. I    !>.    |,  ncli.  <l    \\illi 

ll'il     \\llll   I,      l»\   '\\  Illdl    lln-       .lll|.li:il,       III      lillviT    III   (lllll.nlvcd,    Mini 

III.     Mm. I   :  :iiii|>l<-   l  .  h.  .il.  -I    wild   lr,|...    nl|.liil.'  <>|     ,.,l(|ii,   in  il.- 
Mi  Tllx  1 1   ill  M  >  Vi'.        <  '"in  |  in  l  Hi;;   Ilir   rrillllhi  ill     Ilir   I  III.  .•  11      ,.i  \  ',,   1 1 
I.    r;i    ,ll  y    1 1  ill  III  I    lldW    III  1 1. 'I  I    ill'  I  lir    i  il  I"  I  1 1. 1  I    11,1111  III  II I  III'  Illl  VI 'I    Wll.ll 
I  II  IN'  ,  I    Illlo  II       ill)  .lull-,    Mini   )|i>  W    III  lir  1 1   Mill)  II,  ('I  I  It  trifle. 

Ms  |i"  .nlpliili     <>l    Hodn  l.itlli/ftd       11,  nl'    which    f  i  \ « 

;   ',.  ,  !  '  ,11,1,1,1 

I1'.. i    '.nil. hi. !.•   n|    !;.,<( I;,,!,,,   h  ..     in      i     .iiiln,  in 

.1    h,   in. -II,    111  n.  cnicililc,  n.ml   wli.'ii    hijiii'l  i  nl  i  ...In''. 

•  .mi.-.    .  nl       u!|, I, MI    (  I..  MM    I., n.    ,     ill  ml.  i  \..l  ..    in     .in  ill    |in  cm, 

"i     in-.   Inn.'   | MI    HP    iMHlni"    up    I,,     .nl.  ijtlf         I'oti I     rm    mi 

MM n    [ilnlc  HIM!  <h  .  ...l\  .     in    \\.il.r         II     l.il..    .    i.i'vriiil    l.'.ii    .    I,. 
1'iirc    III'      ''.lull. .n    ;i|,|..  ,n    .    |n  i  li'clly  ch'.ir,    '.h.,uni"    n     \.||..w- 

•  •i. I'll-  iiltnvc   He     l.l.i.-l     Itc.lllii,  nl          \\  IP  n   .|i;i.wii   ..II     II    I  i   i.  ;nly 
IMC   ii-i... 

Tiir  operation    l;,kr..   Irs:;  limr   if  Ilir   Ii  y  po:  ;i  1 1  pi,  i  Te 
:.olnl.on     i  ,     u:.r.|     in     ;,     |,,,|     :,l;,|r.        All    clilondr  of 

.ii\ri     .mil  ulao  Hulphate  of  wilvor,  if  prevent,  in 
2* 


34  ROASTING    OF    ORES. 

dissolved  by  the  hyposulphite  and  carried  off,  be- 
side the  base  metal  chlorides.  The  two  assays, 
when  ready,  are  compared,  and  the  difference 
shows  the  silver  which  was  converted  into  a  chlo- 
ride. For  instance,  if  No.  1  assayed  83  ounces  per 
ton,  and  No.  2  from  the  filter  4  ounces,  the  differ- 
ence, 79,  is  that  part  which  became  chloridized. 
That  is, 

83  :  79  =  100  :  x  —  95  per  cent. 

22.  Toward  the  end  of  the  roasting  very  little, 
if  any,  sulphate  of  silver  will  be  found  in  the  ore; 
but  even  if  a  small  percentage  of  it  should  remain, 
it  may,  for  the  purpose  of  amalgamation  or  ex- 
traction, be  considered  equal  to  chloride  of  silver;, 
for  as  soon  as  it  dissolves  in  water,  it  becomes  a 
chloride,  precipitated  by  the  salt,  of  which  a  part 
is  always  yet  found  undecomposed  in  the  ore.  To 
obtain  a  general  idea  of  the  amount  of  soluble  base 
metal  chlorides  and  sulphates,  it  is  sufficient  to 
put  a  small  sample  of  about  half  an  ounce  on  the 
filter  as  before,  and  to  leach  it  with  hot  water. 
The  leach  obtained  is  tried  again  with  the  sulphide 
of  sodium.  A  thick  precipitate  shows  that  a  large 
amount  of  soluble  chlorides  is  in  the  roasted  ore. 
If  a  reaction  of  copper  is  expressly  desired,  ammo- 
nia should  be  used  in  place  of  the  sulphide  of  so- 
dium. In  presence  of  'much  iron  the  precipitate 
will  appear  brown.  This  precipitate  must  smell 
strongly  of  ammonia.  If  copper  is  present,  a  clear 


ROASTING    OF    ORES.  35 

blue  liquid  will  be  seen  above  the  iron  precipitate 
after  some  time;  or  the  whole  maybe  brought  on  a 
filter  to  separate  the  liquid  from  the  precipitate. 

Means  of  Destroying  Base  Metal  Chlorides. 

23.  It  is  very  difficult  to  get  rid  of  all  the  base 
chlorides.  They  are  formed  under  the  action  of 
chlorine  and  hydrochloric  acid.  The  most  of  the 
metal  chlorides  are  volatile,  and  a  part  is  carried 
off  through  the  chimney.  Another  part  of  the 
chlorides  gives  off  some  of  its  chlorine,  whereby 
sulphates,  undecomposed  sulphurets,  antimonates, 
arsenates  and  free  oxides,  are  chloridized.  Chlo- 
rides which  are  disposed  to  transfer  chlorine  to 
other  metals  in  combination  with  sulphur  or 
arsenic,  are:  the  proto-chloride  of  iron  and  of  cop- 
per, "the  chlorides  of  zinc,  lead  and  cobalt.  When 
in  this  way  the  most  of  the  metals  are  chloridized, 
the  base  metals,  principally  iron  and  copper,  are 
losing  their  chlorine  gradually,  being  first  converted 
into  sub-chlorides  and  then  into  oxides.  The 
roasting  for  this  purpose  must  continue  with  in- 
creased heat,  even  when  the  chlorination  of  the 
silver  is  finished.  At  an  increased  heat,  the  base 
metal  chlorides  lose  their  chlorine,  while  the  chlo- 
ride of  silver  remains  undecomposed,  unless  a  very 
high  temperature  should  be  applied.  This  process 
requires  a  long  time,  consequently  also  more  fuel. 
The  decomposition  of  these  chlorides  is  greatly 
assisted  by  the  use  of  5  to  6  per  cent,  of  carbonate 


36  BOASTING    OF    ORES. 

of  lime  in  a  pulverized  condition.  Lime  does  not 
attack  the  chloride  of  silver,  but  it  is  not  advisable 
to  take  too  much  of  it,  as  it  would  interfere  to 
some  degree  with  the  amalgamation.  The  pulver- 
ized lime  rock  must  be  charged  toward  the  end  of 
the  roasting.  First,  two  per  cent,  is  introduced  by 
means  of  a  scoop,  the  whole  well  mixed,  and  then 
examined  either  with  sulphide  of  sodium  (§  22)  or 
in  the  following  way : 

A  small  portion  of  the  roasted  ore  is  taken  in  a 
porcelain  cup  or  glass,  and  mixed  with  some  water 
by  means  of  a  piece  of  iron  with  a  clean  metallic 
surface.  If  the  iron  appears  coated  red  with  cop- 
per, some  more  lime  must  be  added.  In  place  o2 
iron, — especially  if  no  copper,  but  some  other  base 
metal  is  present, — some  quicksilver  is  mixed  with 
the  sample.  In  the  presence  of  base  metal  chlo- 
rides, the  quicksilver  is  coated  immediately  with  a 
black  skin. 

When  endeavoring  to  expel  the  base  metals  by 
heat,  the  loss  of  silver,  in  presence  of  much  anti- 
mony, lead  and  copper,  should  be  investigated 
very  carefully.  Under  certain  circumstances  it 
is  not  uncommon  to  find  a  loss  of  even  50  per 
cent,  of  the  silver,  if  the  chloridizing  roasting  is 
carried  on  at  a  high  heat  for  a  great  length  of  time. 
The  loss  increases  with  the  duration  of  roasting 
and  with  the  degree  of  temperature.  When  such 
ore  is  under  treatment,  it  is  necessary  to  take  sam- 
ples during  the  roasting,  and  to  examine  the  same 
for  the  amount  of  chloride  of  silver,  and  also  for  its 


BOASTING    OF    ORES.  37 

loss,  and  to  stop  roasting  when  the  highest  per- 
centage of  chloride  of  silver  is  obtained,  without 
reference  to  the  condition  of  base  metals. 

Steam  Decomposes  Base  Metal  Chlorides 
Effectively. 

The  formation  of  base  metal  chlorides  can  be 
avoided  by  a  proper  but  more  expensive  roasting 
(§  33).  It  requires,  first,  an  oxidizing  roasting, 
with  the  application  of  steam.  This  roasting  must 
continue  until  all  the  metals  are  desulphurized  and 
converted  into  oxides.  When  this  is  accomplished, 
salt  and  green  vitriol  are  added,  and  the  roasting 
continued  until  all  the  silver  is  chloridized. 

There  is  also  a  very  good  way  of  getting  out  a 
great  deal  of  the  base  chlorides  of  the  ore  before 
the  silver  is  amalgamated  or  extracted,  by  leaching 
the  ore  with  hot  water  (§  77). 

Application  of  Steam  in  Roasting. 

24.  The  application  of  steam  in  roasting  is  ad- 
vantageous, for  the  reason  that  hydrochloric  acid 
is  created  by  the  decomposition  of  chlorides,  which 
in  turn  decomposes  the  sulphurets.  The  Irydrogen 
decomposes  also  the  chloride  of  silver,  which,  upon 
being  reduced  to  metallic  condition  by  its  affinity 
for  chlorine,  in  turn  decomposes  the  hydrochloric 
acid.  The  silver  may  thus  change  repeatedly  from 
a  metallic  condition  to  a  chloride,  while  the  base 


38  ROASTING    OF    ORES. 

metal  chlorides  are  reduced  to  oxides,  and  in  that 
state  do  not  interfere  with  the  amalgamation  or 
precipitation.  The  application  of  steam,  however, 
requires  a  great  deal  more  fuel  during  the  roast- 
ing. Taking  the  moisture  of  the  fuel  into  consid- 
eration, there  is  no  roasting  done  without  steam, 
although  with  a  limited  quantity. 

Silver  Ore,  Containing  Lead,  Unfit  for  a 
Chloridizing  Roasting. 

25.  Lead  has  a  bad  influence  in  amalgamation 
and  precipitation,  and  even  in  the  roasting  itself, 
causing  a  baking  of  the  ore  at  the  slightest  undue 
rising  of  the  temperature.  The  chloride  of  lead 
amalgamates  easily,  especially  in  iron  pans.  Ores 
with  8  to  15  per  cent,  of  lead  still  allow  of  a  success- 
ful roasting.  A  part  of  the  formed  chloride  of  lead 
escapes  in  gaseous  form,  another  part  is  reduced 
by  degrees  to  oxy-chloride  of  lead.  This  latter 
combination  goes  mostly  into  the  amalgam.  If 
there  is  more  lead  in  the  ore  than  15  per  cent.,  it 
gives  sometimes,  according  to  its  nature,  as  much 
as  85  per  cent,  of  silver,  and  the  retorted  amalgam 
is  submitted  to  cupellation  in  order  to  separate  the 
lead.  In  Hungary  (Offenbanya),  black  copper,  con- 
taining, besides  the  silver,  10  per  cent,  of  lead, 
is  subjected  to  a  chloridizing  roasting.  The 
pulverized  copper  is  mixed  with  12  per  cent,  of 
salt,  1  per  cent,  of  green  vitriol,  and  3  percent,  of 
saltpetre.  The  saltpetre  oxidizes  the  lead  to  a  sul- 


ROASTING    OF    ORES.  39 

phate,  which  is   not   affected  in   the   subsequent 
amalgamation  in  barrels. 

Difference  in  Roasting  Ore  for  Pan  Amal- 
gamation, as  compared  with  that  for 
other  Modes  of  Extraction. 

26.  The  roasting  of  silver  ores,  if  imperfect, 
will  give  a  better  result  by  amalgamating  in  an 
iron  pan,  than  in  wooden  barrels  or  by  precipi- 
tation.    This  is  due  to  the  better  decomposition  of 
sulphates  and  undecomposed  sulphurets  under  the 
grinding   muller.     The  roasting  for  pan  amalga- 
mation is  therefore  less  delicate.     However,  when 
once  at  work,  it  is  always  better  to  do  the  roasting 
properly;  but  it  is  not  necessary  to  sift  the  ore 
after  roasting  in  order  to  separate  the  lumps  from 
the  mass,  as  is  done  with  the  barrel  amalgamation, 
except  to  prevent  nails  from  coming  into  the  pan. 
The  formation  of   such  lumps,  however,  must  be 
avoided  as  much  as  possible.     Imperfect  roasting 
in  the  presence  of  base  metals,  gives  in  pans  always 
a  low  fineness  of  bullion. 

Examples  of  Local  and  Different  Roast- 
ings. 

27.  Boasting  of  Silver  Ores  in  Freiberg  (Saxony) . 
The  amalgamation  process,  and  consequently  this 
kind  of  roasting,  was  given  up  long  ago  at  Frei- 
berg ;  but  the  method  of  roasting  as   performed 


4:0  ROASTING    OF    ORES. 

there  is  nevertheless  very  interesting.  The  ore 
subjected  to  roasting  consisted  of  silver  glance, 
brittle  silver  ore,  ruby  silver,  metallic  silver, 
fahl  ore,  bournonite,  zincblende,  sulphide  of  anti- 
mony, iron,  copper,  and  nickel  pyrites,  and  of 
gangue,  viz:  quartz,  calc,  brown,  heavy,  and  fluor 
spar.  It  contained  from  sixty  to  one  hundred 
ounces  of  silver  per  ton. 

The  dry  crushed  ore  was  first  spread  on  a  plat- 
form ;  on  this  a  layer  of  damp  ore,  from  the  wet 
concentration,  was  laid,  and  then  10  per  cent,  of 
salt.  This  order  was  repeated  from  six  to  eight  times. 
The  stratified  mass  was  mixed  thoroughly  by  means 
of  shovels  and  a  coarse  sieve.  This  mixture  con- 
tained from  9  to  10  per  cent,  of  moisture.  For 
this  reason,  after  a  charge  of  450  to  500  pounds 
was  introduced  through  a  hole  in  the  roof,  the  fire 
was  kept  very  low,  in  order  to  dry  it  at  a  dark  red 
heat,  and  the  ore  was  diligently  raked  by  two  men, 
working  alternately.  As  soon  as  the  decrepitation 
of  the  salt  ceased,  the  ore  was  ridged  from  the 
bridge  toward  the  flue,  through  the  middle  of  the 
furnace,  and  the  formed  lumps  broken  up  by  iron 
hammers  attached  to  long  handles.  After  this  was 
done,  the  heat  was  increased,  whereby  the  ore, 
under  constant  stirring,  assumed  a  red  hot  con- 
dition, and  the  sulphur  commenced  to  burn  quite 
lively.  This  stage  was  reached  in  two  hours  from 
the  beginning.  The  desulphurization  commences 
with  the  burning  of  the  sulphur,  creating  a  tem- 
perature sufficiently  high  to  continue  the  roasting 


ROASTING    OF    ORES.  41 

without  fuel  for  some  time.  Fumes  are  evolved, 
consisting  of  steam,  antimony,  sulphurous  acid, 
arsenic,  etc.  This  desulphurization  takes  again  two 
hours,  the  workmen  all  the  time  raking  and  chang- 
ing the  hotter  part  of  the  ore  at  the  bridge  with 
the  cooler  at  the  flue.  The  temperature  is  now 
raised  to  a  light  red  heat,  the  ore  increases  in 
volume,  emitting  chlorides  of  metals,  chlorine,  hy- 
drochloric acid,  etc.  The  formation  of  the  chlo- 
rides progresses  rapidly,  and  is  finished  in  three- 
quarters  of  an  hour.  The  charge  is  then  drawn 
out.  A  too  long  roasting  would  not  give  an 
equally  good  result,  as  some  silver  might  be  de- 
composed to  the  metallic  state,  which  is  not  so 
readily  amalgamated  as  the  chloride. 

In  what  Condition  the  Metals  are  after 
Roasting. 

28.  After  roasting,  the  silver  is  found  almost 
entirely  converted  into  a  chloride,  but  a  small  part 
may  remain  as  a  sulphate  ;  antimonate  of  silver 
also  is  formed.  The  iron  is  converted  into  an  oxide, 
some  into  sulphate  and  ar senate  and  basic  chlo- 
ride. The  copper  appears  also  oxidized,  with  some 
sub-chloride  and  less  chloride.  Lead  remains  prin- 
cipally as  a  sulphate  and  basic  chloride.  Zinc  is 
oxidized.  Antimony  is  found  as  anthnonates  com- 
bined with  oxide  of  antimony,  and  with  other  basic 
oxides.  Nickel  and  cobalt  remain  as  oxides,  chlo- 
rides and  arsenates,  and  the  arsenic  is  found  as  an 


42  ROASTING    OF    ORES. 

arsenate  combined  with  other  nietals.  Besides 
these  metal  combinations,  there  is  uiidecomposed 
salt,  sulphates  of  soda,  of  lime,  etc.  The  charge 
loses  about  10  per  cent,  of  its  weight,  of  which  a 
part  is  regained  from  the  dust  chambers. 

Antimonate  of  silver  is  considered  the  cause  of 
the  loss  of  "ilver  in  roasting,  as  it  is  not  decom- 
posed in  barrels,  and  consequently  not  amalga- 
mated. There  have  been  no  experiments  made  as 
yet  to  determine  whether  aiitimonate  of  silver  is 
decomposed  in  iron  pans  during  the  amalgamation 
or  not.  Probably  it  is  not,  as  the  natural  com- 
bination of  antimony,  lead  and  silver,  cannot  be 
amalgamated  in  pans. 

39.  Roasting  of  Argentiferous  Copper  Ores.  At 
Arivaca  (Arizona)  the  silver  ores  from  the  Heinzel- 
mann  mine  consisted  principally  of  silver  copper 
glance  (stromeyerite)  with  51  per  cent,  of  silver; 
fahl  ore  with  from  2  to  15  per  cent,  silver,  contain- 
ing also  some  quicksilver ;  zincblende,  galena  and 
other  decomposed  argentiferous  copper  ores.  On 
an  average  the  ore  contained  from  $150  to  $200  per 
ton,  and  from  10  to  15  per  cent,  of  copper.  After 
crushing,  the  ore  was  spread  on  a  platform  covered 
with  8  per  cent,  of  salt,  and  mixed  thoroughly  by 
means  of  shovels.  Eight  hundred  pounds  of  it,  as 
a  charge,  were  introduced  through  the  roof  of  the 
furnace  (which  was  constructed  entirely  of  adobe), 
spread  on  the  hearth,  and,  at  a  dark  red  heat, 
stirred  for  two  hours,  at  the  end  of  which  time  the 


ROASTING    OF    OEES.  43 

flame  was  colored  an  intense  greenish-blue,  and 
considerable  fumes  were  emitted.  The  raking  con- 
tinued for  an  hour  and  a  half  more  at  an  increased 
heat,  and  during  this  time  the  ore  was  moved  three 
times  from  the  bridge  to  the  flue  and  back.  A 
sample  taken  from  the  furnace  at  this  time,  put  on 
a  canvas  filter,  wet  with  salt  solution  and  leached 
with  a  hot  concentrated  solution  of  salt,  gave  a 
clear  liquid,  which,  diluted  with  water,  showed  a 
strong  white  precipitate  of  chloride  of  silver,  mixed 
with  antimony  and  lead ;  but  the  quicksilver, 
treated  with  the  same  sample  of  ore  and  water, 
was  cut  and  blackened  to  a  high  degree.  For  this 
reason,  from  5  to  6  percent,  of  pulverized  lime  was 
thrown  into  the  furnace  by  means  of  a  scoop,  as 
much  as  possible  over  the  whole  surface  of  the  ore, 
and  then  raked  and  stirred  diligently  in  order  to 
finish  the  mixing  in  the  shortest  time.  After  four 
hours  from  the  beginning,  the  temperature  was 
raised  to  a  light  red  heat  for  half  an  hour,  and  the 
roasting  was  finished.  There  were  yet  a  great 
many  base  metal  chlorides  in  the  ore,  but  as  metal- 
lic copper  was  used  in  the  barrels,  the  silver  turned 
out  always  over  900  fine.  The  loss  of  silver  was 
12.5  to  13  per  cent. 

30.  Roasting  of  Copper  Matt.  In  smelting  ar- 
gentiferous copper  ores,  the  process  is  sometimes 
regulated  to  produce  a  sulphide  of  copper,  contain- 
ing silver  and  base  metals,  as  antimony,  arsenic, 
zinc,  iron,  etc.  This  sulphide  of  copper,  or  copper 


44  ROASTING    OF    ORES. 

matt,  was  roasted  formerly  and  smelted  again  to 
produce  black  copper  ;  that  is,  impure  metallic 
copper.  For  the  purpose  of  extracting  the  silver 
therefrom,  the  copper  was  melted  together  with  a 
certain  percentage  of  lead,  and  the  latter,  with  the 
silver,  extracted  by  liquation  and  cupelled.  The 
remaining  copper  contained  still  some  silver  and 
lead,  and  the  process  was  a  very  lengthy  one  be- 
fore finished.  To  avoid  the  liquation,  the  copper 
matt  was  treated  by  amalgamation,  and  the  silver 
extracted  at  once.  For  this  purpose  the  matt  was 
crushed  and  sifted,  and  the  coarse  part  ground. 

Of  this  powdered  matt,  300  pounds  are  charged 
in  a  double  furnace,  of  which  the  upper  hearth 
prepares  the  ore  by  a  moderate  roasting,  while  the 
lower  one  finishes  the  operation  at  a  higher  tem- 
perature. In  each  of  the  hearth-departments  the 
matt  is  treated  for  two  hours  and  a  half.  Silver 
and  the  other  metals  are  first  converted  into  sul- 
phates and  then  mostly  decomposed  to  oxides,  but 
the  silver  remains  for  the  greatest  part  in  metallic 
condition.  The  matt  is  drawn  out,  mixed  with  8 
per  cent  of  salt  and  12  per  cent,  of  lime,  and  with 
salt  water  into  a  paste,  which  is  allowed  to  rest  for 
twelve  or  fourteen  hours.  The  paste  is  then  dried, 
powdered  between  rollers,  and  again  roasted  two 
hours  and  a  half.  During  this  process,  samples 
are  taken  and  mixed  with  water  and  a  few  drops  of 
mercury.  If  this  appears  coated  bluish,  it  proves 
the  presence  of  metallic  salts,  and  some  more  lime 
must  be  added  ;  if  after  this  the  quicksilver  re- 


ROASTING    OF    OKES.  45 

mains  perfectly  white, — parting,  however,  in  many 
minute  globules, — it  proves  that  too  much  of  the 
lime  was  used,  and  in  this  case  some  of  the  first 
roasted  matt  is  added. 

The  purpose  of  wetting  the  roasted  ore,  as  above 
described,  is  the  formation  of  chloride  of  silver. 
As  there  are  always  sulphates  of  the  metals  pres- 
ent after  the  first  roasting,  they  decompose  the 
salt,  and  the  chlorine  acts  on  the  metallic  silver. 
This  process  is  not  perfectly  finished,  and  therefore 
the  second  roasting. 

31.  Roasting  of  Black  Copper.  The  black  cop- 
per obtained  from  smelting  (in  Schmoellnitz,  Hun- 
gary) contains  from  110  to  150  ounces  of  silver  per 
ton,  and  85  to  89  per  cent,  of  copper.  To  pulver- 
ize this  it  must  be  made  red  hot  in  a  reverberatory 
furnace  and  crushed  while  red  hot.  The  powder 
must  be  sifted  and  then  ground  fine.  The  pulver- 
ized metal  is  then  mixed  with  7  to  9  per  cent,  of 
salt,  and  roasted  in  the  usual  way  for  six  to  six 
and  a  half  hours,  in  charges  of  400  pounds  each. 
No  green  vitriol  is  added  for  the  purpose  of  de- 
composing the  salt ;  and  as  there  is  not  more, than 
from  \  to  1  per  cent,  of  sulphur  in  the  black  cop- 
per, the  salt  decomposes  through  direct  action  on 
the  copper.  First,  chloride  and  subchloride  of 
copper  are  formed.  The  copper  chloride  transfers 
chlorine  to  the  metallic  silver,  and  is  reduced  to  a 
sub-chloride. 

In  other  places  iron  pyrites  are  added  to  the 


46  ROASTING    OF    ORES. 

black  copper,  by  which  the  chlorination  is  pro- 
moted. At  Oriklowa  (Banat)  5  per  cent,  of  iron 
pyrites  and  12  per  cent,  salt  are  used,  roasting 
twelve  hours.  The  loss  of  silver  is  7  percent.,  and 
of  copper  3  per  cent.,  during. the  roasting.  The 
expense  of  roasting  is  $7.30  per  ton. 

32.  Roasting  of  Silver  Ore  at  Flint,  Idaho  Ter- 
ritory, in  O'Hara's  Mechanical  Furnace.  The  ore 
from  the  Rising  Star  mine,  at  Flint,  contains  ar- 
gentiferous fahl  ore,  miargyrite,  ruby  silver,  zinc- 
blende,  galena,  iron  pyrites  and  sulphide  of  anti- 
mony. On  an  average  the  ore  paid  between  $90 
and  $100  per  ton,  containing  some  gold.  The 
gangue  is  quartz.  It  is  crushed  through  sieves  with 
forty  holes  to  the  inch,  together  with  5  per  cent, 
of  salt.  The  furnace  (§  57,  Fig.  8)  is  charged  con- 
tinually by  machinery  at  one  end,  a.  The  ore  is 
moved  by  degrees  forward,  and  arriving  at  the  first 
fire-place,  c,  commences  to  disengage  sulphur. 
Between  this  fire-place  and  the  second,  which  is  on 
the  opposite  side,  between  c  and  d,  the  chlorination 
begins  at  an  increased  heat.  The  flame  shows 
partly  a  blue  color,  originating  from  chloride  of 
copper,  and  white  fumes  are  also  evolved.  Be- 
tween the  second  and  the  third  fire-place,  d,  the 
chlorination  is  finished  at  a  light  red  heat.  From 
the  cooling  hearth,  e,  the  roasted  ore  is  continually 
discharged  on  the  dump,/*.  It  takes  six  hours  be- 
fore the  ore  from  the  feeding  place,  a,  arrives  at 
the  dump.  Although  not  more  than  5  per  cent,  of 


ROASTING    OF    ORES.  47 

salt  is  added,  the  roasted  ore  contains  about  90  per 
cent,  of  the  silver  converted  into  a  chloride.  The 
gases,  containing  free  chlorine  and  chloride  com- 
binations, emitting  chlorine  (§16),  being  in  contact 
with  the  surface  of  the  ore  while  passing  over  it 
for  a  space  of  eighty  feet,  have  a  chloridizing  influ- 
ence on  it,  replacing  thus  a  certain  amount  of  salt. 
These  three  furnaces  roasted  twenty  tons  of  ore 
in  twenty-four  hours.  The  expenses  were  as  fol- 
lows : 

For  wood,  five  cords,  at  $5 $  25  00 

For  four  men  at  the  furnaces,  at  $4 16  00 

For  two  men  bringing  in  wood,  etc.,  at  $4. .       8  00 

For  one  man  as  watch,  in  the  night 4  00 

For  blacksmith  work 5  00 

For  2,000  pounds  of  salt,  at  8c 160  00 

Total  expense $218  00 

or  $10.90  per  ton.  The  capacity  of  the  three  fur- 
naces is  calculated  for  more  than  twenty  tons. 
Each  one  could  easily  treat  ten  tons  of  the  Rising 
Star  ore  in  twenty-four  hours.  The  roasted  ore 
was  treated  by  amalgamation  in  pans,  applying  the 
"  leaching  process." 

Taking  moderate  prices  of  salt  and  labor  into  cal- 
culation, instead  of  the  "  Flint  Tariff,"  the  roasting 
expense  in  O'Hara's  furnaces  would  not  exceed 
$5.50  per  ton,  provided  three  furnaces  were  at  work. 

33.  Roasting  of  Silver  Ores  for  the  Patera  Pro- 
cess. The  ores  treated  by  Patera's  process  (§  71) 
at  Joachim  sthal  are  remarkable  for  the  numerous 
mineral  species  occurring  in  the  ore.  Among  these 


48 


ROASTING    OF    ORES. 


may  be  mentioned  silver,  lead,  different  compounds 
of  copper,  bismuth,  iron,  uranium,  nickel,  cobalt, 
etc.,  sulphur,  arsenic  and  antimony.  Before  the 
introduction  of  Patera's  process,  the  extraction  of 
silver,  on  account  of  so  many  base  metals,  was  very 
difficult.  The  success  of  Patera  was  not  so  much 
in  adopting  hyposulphites,  as  proposed  by  Percy 
and  Hauch,  but  in  his  modification  of  the  roasting 
process,  by  which  only  the  silver  was  converted 
into  a  chloride. 

The  pulverized  ore  is  placed  in  the  furnace,  in 
charges  of  from  four  to  five  hundred  pounds. 
First  quite  a  moderate  heat  is  applied,  and  gradu- 
ally increased,  but  not  so  much  as  to  induce  clot- 
ting. As  soon  as  the  ore  appears  red  hot,  steam  is 
admitted,  with  about  four  pounds  pressure  to  the 
square  inch.  As  the  steam  consumes  heat,  more 
fuel  must  be  used  to  keep  up  a  red  heat.  The  ore 
must  be  constantly  raked  during  the  whole  period 
of  this  roasting.  It  takes  about  four  hours  to 
finish  this  process,  after  which  the  ore  is  drawn 
and  permitted  to  cool.  The  iron  appears  now  as 
an  oxide — also  the  copper  ;  some  sulphate  of  cop- 
per is  also  present,  and  the  silver  is  principally  in 
the  state  of  a  sulphate. 

The  oxidized  ore  is  now  ground  finer,  mixed  with 
from  5  to  12  per  cent,  of  common  salt,  and,  at  the 
same  time,  with  2  to  3  per  cent,  of  calcined  green 
vitriol.  This  mixture  is  spread  upon  the  hearth  in 
the  furnace,  and  subjected  to  a  second,  now  chlo- 
ridizing,  roasting.  It  takes  about  an  hour  before  a 


ROASTING    OF    ORES.  49 

red  heat  is  reached.  Steam  is  then  introduced,  as 
above,  under  continuous  stirring.  The  fire  is  gradu- 
ally increased,  and  the  roasting  finished  within  from 
five  to  eight  hours,  according  to  the  value  of  the 
ore.  There  are  condensing  chambers  for  catching 
volatile  metals  and  ore  dust.  They  are  of  impor- 
tance if  rich  ore  is  treated,  and  without  this  con- 
trivance several  per  cent,  of  silver  would  be  lost. 
When  finished,  the  ore  is  drawn  and  allowed  to  lie 
undisturbed  for  some  time,  after  it  has  been  moist- 
ened. In  this  condition  the  chlorination  con- 
tinues. The  application  of  steam  causes  nearly 
twice  the  consumption  of  fuel,  but  it  has  been  shown 
that,  by  the  steam,  hydrochloric  acid  is  formed, 
whereby  arsenic  and  antimony  are  expelled,  and  at 
the  same  time  the  chlorination  of  the  silver  greatly 
favored.  Over  one  ton  of  coal  and  one-half  cord 
of  wood  are  consumed  for  each  ton  of  ore  roasted 
in  this  way. 

34.  Roasting  for  Augustin's  Process.  To  this 
process  principally  matt  is  subjected.  The  process 
requires  a  chloridizing  roasting.  The  ore,  if  not 
itself  rich  in  sulphurets,  is  mixed  with  iron  py- 
rites, slag,  lime,  etc.,  and  smelted.  The  molten 
sulphide  of  iron  takes  up  the  silver  and  deposits 
itself  below  the  slag  on  the  bottom  of  the  furnace. 
The  silver  is  thus  separated  from  the  earthy  part 
and  concentrated  in  the  matt.  Argentiferous  cop- 
per ores  are  likewise  smelted  for  the  purpose  of 
obtaining  argentiferous  copper  matt.  The  matt  is 
3 


50  ROASTING    OF    ORES. 

then  finely  pulverized,  and  400  pounds  of  it  are  in- 
troduced into  the  furnace.  The  roasting  goes  on  now 
in  the  usual  way,  by  starting  with  a  moderate  tem- 
perature, gradually  increasing  it,  exposing  every 
portion  of  the  ore  to  the  intense  heat  by  frequent 
stirring,  etc.  At  the  end  of  eight  hours  the  roast- 
ing is  generally  completed,  the  matt  looks  dark 
and  earthy,  and  no  fumes  of  sulphurous  acid  can 
be  perceived.  The  roasted  stuff  is  now  drawn  out 
and  permitted  to  cool.  After  this  the  matt  powder 
is  ground  finer,  sifted  or  bolted,  and  given  back  to 
the  furnace. 

Four  hundred  pounds  are  mixed  with  5  per  cent, 
of  salt.  Sulphates  are  present,  but  oxides  of 
metals  predominate  in  the  mixture.  The  formation 
of  the  chlorides  commences  immediately,  and  the 
roasting  is  concluded  after  one  or  two  hours.  The 
temperature  in  this  second  Coasting  is  kept  low,  as 
the  smelting  of  the  chloride  of  silver  must  be  pre- 
vented—for when  melted  the  chloride  of  silver 
dissolves  with  more  difficulty  in  a  salt  solution.  In 
other  places,  after  the  first  roasting,  the  mass  is 
not  taken  out  and  re-ground,  but,  when  finished? 
only  a  portion  is  drawn,  mixed  with  from  1  to  6 
per  cent,  of  salt,  according  to  the  purity  of  the 
matt,  charged  again,  mixed  with  the  balance  of  ore 
in  the  furnace,  and  roasted  for  one-half  to  three- 
quarters  of  an  hour. 

35.  Boasting  of  Silver  Ores  for  the  Chlorination 
Process  (§  74).  It  is  not  absolutely  necessary  for 


BOASTING    OF     ORES.  51 

this  process  to  have  the  ore  roasted  with  salt,  but 
it  has  been  found  that,  on  account  of  different 
earths,  an  addition  of  1  or  2  per  cent,  of  salt  pro- 
duces a  better  result.  This  process  extracts  cop- 
per, gold  and  silver,  each  of  which  is  obtained 
separately;  but  it  makes  a  difference  in  roasting, 
whether  the  copper  is  intended  to  be  saved  or  not, 
as  in  many  localities  the  copper  is  at  present  of  no 
value,  or  the  old  iron  for  precipitation  is  too  ex- 
pensive. 

The  ore  is  crushed  dry  through  a  sieve  of  -forty 
holes  to  the  running  inch.  Some  ore  allows  also 
thirty  holes  to  the  inch.  In  case  the  ore  contains 
so  much  clay  or  talc  that  no  leaching  is  admis- 
sible, the  ore  is  crushed  wet,  separated  from  slime, 
and  dried .  Eight  hundred  to  one  thousand  pounds 
are  charged,  and,  according  to  the  quality  of  the 
ore,  the  heat  raised  quickly  or  slowly  to  a  bright 
red  heat.  This  is  an  oxidizing  roasting,  conse- 
quently much  stirring  is  required.  Ores  with  but 
few  sulphurets  appear  sufficiently  well  roasted 
after  three  hours  ;  other  ores  containing  an  abun- 
dance of  sulphurets,  take  from  five  to  six  hours  and 
more  before  all  sulphur,  arsenic  and  antimony  are 
expelled ;  and  the  arsenates  and  antimonates 
formed  by  the  two  last  are  not  volatile.  When  de- 
sulphurized, 1  or  2  per  cent,  of  salt  is  thrown  in 
the  furnace  and  mixed  with  the  ore  as  intimately 
as  possible.  Three-quarters  of  an  hour  after  the 
addition  of  the  salt,  and  with  only  a  moderate  heat, 


52  ROASTING    OF    ORES. 

the  roasting'  is  finished.  Ores,  not  rich  in  sul- 
phurets,  may  be  mixed  with  salt  when  charged. 

If  it  is  intended  to  extract  the  copper  also,  this 
must  be  transformed  into  chloride  of  copper.  To 
accomplish  this,  two  things  must  be  observed, — 
first,  no  oxide  of  copper  should  be  formed  during 
the  roasting  ;  and  second,  more  salt  must  be  used. 
Stoichiometrically,  each  pound  of  copper  requires 
1.84  pounds  of  salt  to  form  a  chloride,  provided 
all  chlorine  is  taken  up  by  the  copper  ;  but  as 
this  is  not  the  case,  as  a  great  deal  of  chlorine  is 
also  absorbed  by  other  metals,  etc.,  it  follows  that 
at  least  two  pounds  of  salt,  if  not  more,  must  be 
taken  for  each  pound  of  copper  in  the  ore.  For 
most  localities  such  a  quantity  of  salt  could  not  be 
used  on  account  of  the  difficulty  in  obtaining  it. 
It  may  be  mentioned  here  that  if  the  brine,  remain- 
ing after  the  copper  has  been  precipitated  by  iron, 
should  be  condensed  by  evaporation,  exposing  it  to 
the  heat  of  the  sun,  which  might  be  practicable  on 
the  Pacific  Coast  in  the  dry  season,  the  condensed 
salt,  consisting  of  chloride  of  iron,  could  be  added 
to  the  ore  as  a  chloridizer,  whereby  a  considerable 
percentage  of  salt  would  be  saved. 

In  roasting  with  salt  with  reference  to  extracting 
the  copper,  the  ore  is  first  roasted  for  itself  at  a 
low  temperature,  so  as  not  to  decompose  the  sul- 
phates by  a  bright  red  heat,  but  long  enough  to 
decompose  all  sulphurets..  "When  this  is  accom- 
plished, the  salt  is  introduced  and  the  roasting 
finished  in  one-half  to  three-quarters  of  an  hour 
thereafter. 


ROASTING    OF    ORES.  53 

36.  Roasting  of  Silver  Ores  in  a  Long  Furnace  at 
San  Martial,  Sonora,  Mexico.  For  the  purpose  of 
roasting  silver  ores  for  the  Solving  and  Precipi- 
tation Process,  there  are  several  long  furnaces 
(thirty  feet  long)  built  up  by  Mr.  O.  Hofmann,  in 
Sonora.  Using  long  furnaces  is  'found  a  great 
economy  in  every  respect.  A  great  advantage  re- 
sults also  from  moving  the  ore,  at  intervals  of  from 
one  to  three  hours,  from  one  hearth  to  the  other. 
By  this  means  it  is  impossible,  even  with  careless 
roasters,  to  find  raw  ore  in  the  finished  charge,  as 
would  happen  under  such  circumstances  if  the  cor- 
ners of  single  roasting  furnaces  wrere  not  very  care- 
fully attended  to  during  the  roasting. 

The  furnace  at  San  Marcial  is  sixty  feet  long. 
The  plan  was  made  by  Mr.  Graff,  who  superin- 
tended the  reduction  works.  It  is  a  level  hearth 
sixty  feet  long,  representing  six  furnaces,  each  ten 
feet  long,  parted  only  by  the  projecting  wall  in- 
side, as  shown  by  v,  Figs.  6  and  7  (§  49).  There 
are  six  working  doors  on  one  side,  and,  on  account 
of  the  length,  an  auxiliary  fire-place  is  placed  at  the 
back  side,  before  the  last  two  hearths.  Each  hearth 
contains  800  pounds  of  ore,  and  is  attended  by  two 
Yaque  Indians  at  a  time,  stirring  alternately. 
These  twelve  Indians  perform  all  the  work  about 
the  furnace,  carry  the  wood  from  the  adjoining- 
yard,  split  what  is  too  thick,  carry  out  the  ashes, 
etc.  The  ore  on  the  first  hearth,  nearest  to  the 
fire-place,  has  always  a  light  red  heat,  which  de- 
creases with  the  distance  from  the  bridge,  so 


54  ROASTING    OF    ORES. 

that  the  fifth  appears  quite  dark  if  not  assisted 
by  the  second  fire-place.  After  stirring  one  hour 
on  all  six  hearths,  the  charge  from  the  first  is 
drawn  out  through  a  door  in  the  rear,  on  a  large , 
smooth  platform,  arid  immediately  spread  by  means 
of  shovels  in  a  layer  one  or  one-and-a-half  inches 
thick,  so  as  to  have  it  cool  enough  for  transporta- 
tion to  the  sifting  apparatus  after  the  lapse  of  one 
hour.  As  soon  as  the  hearth  is  cleared,  the  ore  on 
the  second  hearth  is  moved  over  to  the  first,  then 
that  on  the  third  to  the  second,  and  so  on,  till 
the  sixth  hearth  remains  empty  and  is  charged 
through  the  funnel  in  the  roof  with  a  new  charge. 
It  will  be  seen  that  800  pounds  of  ore  are  drawn 
out  every  hour,  and  that  each  charge  is  exposed  to 
the  fire  for  six  hours.  It  is  thus  evident  that, 
being  moved  six  times  from  one  hearth  to  the  other, 
the  ore  arrives  perfectly  prepared  to  the  finishing 
heat.  After  roasting  and  sifting,  the  ore  is  amal- 
gamated in  pans,  but  as  it  contains  some  carbonate 
and  sulphuret  of  lead  the  amalgam  is  charged  with 
base  metals,  so  much  that  refining  by  cupellatioii 
is  necessary.  From  8  to  10  per  cent,  of  salt  is 
added  and  mixed  with  the  ore  before  it  is  charged. 
Preparations  are  made  now  to  introduce  the  solv- 
ing and  precipitation  process,  if  successful  on  that 
kind  of  ore. 

The  above  arrangement,  employing  so  many 
hands,  is  considered  by  Mr.  Graff  a  local  neces- 
sity; the  Indians  are  cheap,  but  not  very  attentive, 
laborers. 


ROASTING    OF    ORES.  55 

According  to  an  analysis  made  by  Mr.  Graft", 
the  roasted  ore  from  single  furnaces  (treating  $100 
ore)  contained  5  per  cent,  less  of  chloride  of  silver 
than  that  from  a  long  one.  Long  roasting  furnaces 
are  especially  adapted  for  roasting  sulphurets  con- 
taining gold.  Concentrated  sulphurets,  or  ore  con- 
taining an  abundance  of  sulphurets,  allow  the  use 
of  a  very  long  furnace,  with  only  one  fire-place, 
on  account  of  the  heat  created  by  the  burning 
sulphur. 

The  roasting  expenses  at  San  Marcial,  with  the 
furnace  sixty  feet  long,  are  as  follows  : 

2i  men,  day  and  night,  at  50c $12  00 

2  cords  of  wood  at  $3 6  00 

8  per  cent,  of  salt=l,536  Its.  at  2c 30  72 

Kepairs,  etc 3  00 

Total  expense  on  9.8  tons $51  72 

or  $5  27  per  ton. 

A  furnace  thirty  feet  long,  with  the  same  kind  of 
laborers,  800-pound  charges,  drawing  every  two 
hours, — that  is,  4.8  tons  in  24  Ijours,  shows  the 
following  expenses : 

8  roasters  at  50c $  4  00 

1%  cords  of  wood  at  $3 4  50 

8  per  cent,  of  salt  at  2%c 19  20 

Other  expenses 2  00 

Total $29  70 

or  $6  18  per  ton. 

37.  Roasting  in  Stetefeldt's  furnace  at  Reno, 
Nev.  The  mechanical  part  of  the  roasting  itself, 


50  ROASTING    OF    ORES. 

in  tliis  furnace,  is  the  simplest  of  all,  and  also  the 
shortest.  The  finely  pulverized  ore,  mixed  with 
salt,  is  sifted  continuously  by  a  mechanical  ar- 
rangement into  a  shaft.  This  shaft,  a,  (Fig.  9, 
§58),  is  about  twenty-five  feet  high,  and  heated  by 
two  fire-places  provided  with  grates.  The  ore, 
falling  through  the  heated  shaft,  undergoes  chlo- 
rination, — a  process  requiring  only  a  few  seconds. 
After  the  roasted  ore  has  accumulated  on  the  bot- 
tom of  the  shaft  to  the  amount  of  about  1,000 
pounds,  it  may  be  drawn  out.  The  amount  of 
salt  needed  for  chlorination,  varies  according  to 
the  ore;  generally  about  6  per  cent,  or  120  pounds 
to  the  ton,  is  taken  ;  or  even  less,  especially  in 
treating  poor  ores,  when  half  of  that  amount  may 
be  sufficient  in  most  cases.  A  furnace  having  a 
capacity  of  from  fifteen  to  twenty  tons  in  twenty- 
four  hours,  consumes  from  two'  to  three  cords  of 
wood.  In  twenty-four  hours  there  are  employed : 
Two  men  attending  the  feeding  and  conveying  ma- 
chinery, three  firemen,  and  three  men  to  draw  and 
cool  the  roasted  ore.  As  the  latter  three  have 
time  enough  to  carry  the  ore  to  the  pans,  only  half 
of  their  time  should  be  charged  to  the  roasting 
expenses.  According  to  these  figures,  the  total 
expense  of  roasting,  in  Reno,  is  not  more  than — 

For  labor  of  6%  men  at  $3 $19  50 

For  wood,  2%  cords  at  $6 15  00 

Salt,  1,800  pounds  at  l%c 27  00 


Total  expense  on  15  tons $61  50 

or  $4  10  per  ton.     From  88  to  92  per  cent,  of  the 


ROASTING    OF    ORES.  57 

silver  contained  in  the  ore  is  converted  into  a  chlo- 
ride. Of  the  dust  in  the  dust-chambers,  the  silver 
was  found  in  the  state  of  a  chloride  up  to  96  per 
cent.  [See  Scientific  Press,  1869,  p.  377]. 

It  is  evident  that  with  an  improper  treatment  of 
the  fire,  by  using  too  much  or  too  little  fuel,  a  less 
favorable  result  would  be  obtained.  In  the  first 
place,  if  the  temperature  is  kept  too  high,  a  part 
of  the  chloride  of  silver  is  reduced  to  a  metallic 
state,  which,  for  the  purpose  of  amalgamation,  is 
not  so  very  injurious  (§8);  but  the  metallic  silver 
is  a  total  loss  with  the  solving  process.  On  the 
other  hand,  if  there  is  not  sufficient  heat,  some  of 
the  sulphurets  may  remain  undecomposed.  In 
either  case  the  responsibility  is  with  those  in 
charge  of  the  furnace;  but  there  is  nothing  easier 
than  to  keep  up  a  proper  and  uniform  heat  in 
Stetefeldt's  furnace,  there  being  no  other  hand- 
work about  it,  and  all  the  attention  of  the  fireman 
being  directed  to  this  single  point. 

Stetefeldt's  furnace  has  been  compared  some- 
times with  Gerstenhoefer's  shelf  furnace.  In  ref- 
erence to  this  the  Engineering  and  Mining  Journal 
says  : 

' '  Since  the  discovery  and  exploration  of  the  num- 
berless mineral  deposits  in  the  Western  States 
and  Territories,  no  branch  in  metallurgy  has  re- 
ceived so  much  attention  as  the  process  of  roast- 
ing ores  of  all  descriptions.  One  can  hardly  look 
over  a  file  of  Mining  Journals,  or  newspapers  from 
some  mining  district,  without  finding  descriptions 
3*  * 


58  ROASTING    OF    OKES. 

of  new  devices  for  roasting  ores, — all  of  .which 
claim  to  surpass  everything  else  in  this  line 
which  was  known  before.  The  devices  are  as 
strange  as  they  are  many,  and  much  time  and 
money  has  been  wrasted  to  test  impracticable  in- 
ventions. Indeed,  the  high  expense  which  the 
roasting  in  the  old  reverberatory  furnace  entails, 
was  a  strong  inducement  to  invent  some  cheaper, 
and  at  the  same  time  more  effective,  method. 
This  is  especially  of  importance  where  silver  ores 
are  found,  which  require  a  chloridizing  roasting 
preparatory  to  their  amalgamation.  In  such  cases 
the  expense  of  roasting  is  frequently  more  than 
one-half  of  the  total  expense  of  reduction,  and 
consequently  low-grade  ores  cannot  be  worked 
with  a  profit.  •  But  in  spite  of  the  necessity  to 
adopt  some  improved  and  more  economical  pro- 
cess of  roasting,  it  has  been  extremely  difficult  to 
introduce  two  inventions,  which  are  based  upon 
the  most  simple  and  rational  principles — so  sim- 
ple, indeed,  that  it  seems  impossible  to  simplify 
them  any  more.  We  speak  of  the  Gerstenhoefer, 
or  Terrace  furnace,  first  introduced  about  six  years 
ago  at  Freiberg,  and  the  Stetefeldt  furnace,  in- 
vented three  years  ago  at  Austin,  Nevada,  but  first 
introduced  for  regular  working  at  the  mill  of  the 
Nevada  Silver  Mining  Company,  near  Reno,  Ne- 
vada, in  October  of  last  year.  The  nature  of  these 
inventions  can  be  expressed  as  follows : 

Gerstenhoefer   discovered  that    sulphurets    are 
completely  roasted  or  oxjjlized  if  they  fall  against 


ROASTING    OF    ORES.  59 

a  current  of  hot  air  rising  in  a  shaft,  which  is 
filled  with  shelves,  so  as  to  check  and  retard  the 
fall  of  the  ore  particles  at  certain  intervals. 

Stetefeldt  discovered  that  silver  ores,  no  matter 
in  what  combination  the  silver  occurs,  mixed  with 
salt,  are  completely  chloridized  if  they  fall  against 
a  current  of  hot  air  rising  in  a  shaft  with  no  ob- 
structions whatever  to  check  or  retard  the  fall  of 
the  ore  particles. 

It  is  a  matter  of  course  that  in  both  cases  a  cer- 
tain degree  of  fineness  is  required  for  the  ore  to 
be  treated,  and  that  a  much  coarser  material  can 
be  successfully  roasted  in  the  Gerstenhoefer  fur- 
nace than  in  Stetefeldt's. 

We  do  not  intend  to  enter  here  into  a  detailed 
description  of  the  Gerstenhoefer  furnace,  since 
that  invention  has  been  frequently  laid  before  the 
public  in  several  mining  papers;  but  we  will 
merely  compare  it  with  the  Stetefeldt  furnace, 
and  point  out  the  distinctions  of  the  two  inven- 
tions. 

As  a  cheap  chloridizing  roasting  is  a  vital  ques- 
tion for  the  industry  of  silver  mining  in  this  coun- 
try, it  is  evident  that  Stetefeldt's  discovery  far 
surpasses  that  of  Gerstenhoefer  in  importance. 
But  the  question  arises,  whether  the  former,  as 
constructed  by  Gerstenhoefer,  cannot  be  used  as 
well  for  chloridizing  as  desulphurizing  roasting? 
"We  answer,  no.  In  the  Gerstenhoefer  furnace  only 
such  ores  can  be  successfully  treated,  which,  at  a 
red  heat  during  roasting,  have  no  tendency  to 
sinter  or  stick  together. 


60  ROASTING    OF    ORES. 

But  the  small  particles  of  a  charge  of  ore  mixed 
with  salt  are  exactly  in  such  a  condition  while 
roasting,  as  to  have  the  greatest  possible  inclina- 
tion to  sinter  and  adhere  to  the  shelves.  Tfiey 
would  thus  soon  obstruct  the  whole  shaft,  and 
prevent  any  further  wrork.  This  has  been  demon- 
strated by  actual  experiments  on  a  working  scale. 

From  the  foregoing,  it  is  apparent  that  the  ap- 
plication of  the  Gerstenhoefer  furnace,  even  for 
desulphurizing  purposes,  is  very  limited,  and  that 
certain  classes  of  ore  must  be  entirely  excluded 
from  it.  This  is  especially  the  case  with  galena 
ores,  which  are  the  most  expensive  to  roast  in  re- 
verberatory  furnaces. 

In  Stetefeldt's  opinion,  the  shelves  in  the  Ger- 
stenhoefer furnace  are  perfectly  superfluous,  and  all 
ores,  even  galena,  can  be  desulphurized  by  drop- 
ping them  through  a  plain  shaft  heated  by  fire- 
places below,  if  they  are  reduced  to  a  sufficient  de- 
gree of  fineness.  The  escape  of  unroasted  dust 
from  the  shaft  is  of  no  consequence,  as  a  separate 
fire-place  is  constructed  for  the  roasting  of  these 
suspended  particles  in  the  Stetefeldt  furnace. 
Furthermore,  the  feeding  machinery  of  the  Stete- 
feldt furnace  is  based  upon  a  principle  entirely 
differing  from  that  used  with  the  Gerstenhoefer 
furnace. 

That  a  furnace  without  shelves  is  cheaper  and 
easier  to  construct,  more  durable,  less  liable  to  get 
out  of  order,  and  that  it  requires  less  labor  and 
skill  to  run  it,  must  be  conceded  by  everybody. 


ROASTING    OF    ORES.  61 

Much  difficulty  was  experienced  to  provide  suit- 
able feeding  machinery  for  the  Stetefeldt  furnace. 
Gerstenhoefer's  apparatus,  consisting  of  fluted 
rollers,  which  force  the  ore  through  slits  in  the  top 
of  the  furnace,  would  not  answer  at.  all.  The  ore 
fell  down  in  lumps,  and  arrived  at  the  bottom  of 
the  shaft  almost  raw.  The  reason  for  this  be- 
havior is  simply  the  tendency  of  the  particles  of 
all  finely-pulverized  mineral  substances  to  adhere 
to  each  other  if  a  slightly  compressed  mass  of  them 
falls  through  the  air.  It  is,  therefore,  necessary 
to  introduce  the  ore  pulp  so  finely  divided,  that  all 
the  particles  can  be  penetrated  by  the  heat  within 
the  short  time  of  their  fall  through  the  shaft.  To 
feed  the  pulp  with  a  blower,  as  it  is  done  in  Keith's 
desulphurizing  furnace,  was  not  considered  desir- 
able for  the  following  reasons : 

1.  The  fall  of  the  ore  would  be  accelerated. 

2.  The  draft  of  the  fire-places  would  be  im- 
peded by  the  downward  current  of  the  air  from  the 
blower. 

3.  The  formation  of  dust  would  be  considerably 
increased. 

The  feeding  machinery  in  its  present  shape  can 
be  briefly  described  as  follows: 

A  hollow  cast  iron  frame,  kept  cool  by  a  small 
stream  of  water,  rests  on  top  of  the  furnace. 
In  this  frame  is  inserted  a  cast  iron  grate,  which 
is  covered  by  a  punched  screen  of  Russia  iron, 
No.  0  for  wet  crushing,  of  the  trade.  Close  to  the 
punched  screen  moves,  inside  of  the  hopper,  a 


62  ROASTING    OF    ORES. 

coarse  wire  screen,  No.  3  of  the  trade,  which  is 
fastened  to  a  frame.  The  frame  has  flanches  rest- 
ing upon  adjustable  friction  rollers  outside  of  the 
hopper,  and  receives  its  motion  from  a  crank,  with 
If-inch  eccentricity.  To  avoid  the  motion  of  a 
stratum  of  pulp  with  the  coarse  screen,  a  number 
of  thin  iron  blades  are  so  arranged  across  the  hop- 
per that  their  lower  edges  reach  close  down  to  the 
coarse  screen  and  keep  the  pulp  in  place.  When 
the  crank  is  set  in  motion,  the  meshes  of  the 
coarse  wire  screen  cut  through  the  pulp  and 
drive  it  through  the  openings  of  the  punched 
screen.  In  this  way  the  ore  is  introduced  in  a 
continuous  stream  into  the  furnace.  The  motion 
of  the  crank-shaft  was  variably  tried  in  Reno,  at 
from  thirty  to  seventy  revolutions  per  minute." 

The  very  satisfactory  result  of  roasting  silver 
ores  in  Stetefeldt's  furnaces  at  Reno,  induced  the 
Manhattan  Silver  Mining  Co.  to  adopt  the  same  in 
their  mill  at  Austin,  Nevada.  Mr.  Stetefeldt  altered 
the  plan  of  the  Reno  furnace  somewhat,  omitting 
a  dust-chamber  between  the  furnace  and  the  verti- 
cal flue.  Another  important  improvement  is  the 
application  of  gas  generators  (§  58)  in  place  of  the 
arrangement  for  the  use  of  firewood.  The  gen- 
erators are  fed  with  charcoal.  After  the  proportion 
of  air  (which  is  admitted  through  a  separate  flue 
and  is  necessary  for  the  burning  of  the  carbonic- 
oxide  gas)  has  been  regulated,  the  uniform  heating 
of  the  furnace  does  not  depend  on  the  skill  of  the 
fireman.  There  are  three  gas  generators  at  the 


ROASTING    OF    ORES.  63 

furnace,  two  communicating  with  the  furnace 
shaft,  a,  of  Fig.  9,  §  58,  and  one  with  the  vertical 
flue,  b.  One  separate  gas  generator  could  supply 
all  three  entrances,  but  Mr.  Stetefeldt  prefers  to 
have  them  separate,  partly  on  account  of  dispens- 
ing with  iron  gas  pipes  and  other  inconveniences 
connected  with  a  general  generator.  The  choice 
of  firewood  or  gas  arrangement  depends  on  local 
circumstances — the  comparative  price  of  wood  and 
charcoal,  etc. 

The  furnace,  as  represented  in  Fig.  9,  §  58,  is 
calculated  to  roast  from  twenty-five  to  thirty  tons 
of  ore  in  twenty-four  hours,  at  a  cost  of  from  $5  to 
$6  per  ton,  while  the  expenses  in  usual  reverbe- 
ratory  roasting  furnaces  at  Austin  amount  to  $12 
or  $14  per  ton. 

Stetefeldt 's  furnace  is  of  vital  importance,  espe- 
cially for  poor  ores  requiring  roasting.  Consider- 
ing the  temporary  repairs  of  the  arch  and  floor  of 
the  reverberatory  furnaces,  the  amount  of  black- 
smith's work  on  hoes  and  rakes,  besides  what  was 
stated  above,  the  superiority  of  Stetefeldt's  furnace 
over  the  reverberatory  and  other  mechanical  fur- 
naces is  obvious. 

It  may  be  remarked  that  wherever  the  use  of  the 
solving  process  (§  60)  appears  admissible  on  silver 
ores,  this,  in  connection  with  Stetefeldt's  roasting, 
will  allow  the  most  economical  extraction  of  silver, 
even  from  very  rebellious  ores.  The  baking  of  the 
ore  during  the  chlorination,  in  the  presence  of 
lead,  cannot  take  place  in  Stetefeldt's  furnace,  and 
it  is  therefore  very  probable  that  a  higher  amount 


64  ROASTING    OF    ORES. 

of  lead  will  be  less  injurious  than  in  any  other 
roasting  process. 

Considering  the  old,  or  rather  the  usual 
theory  deduced  from  the  roasting  process  in  com- 
mon reverberatoiy  furnaces,  that  sulphates  must 
be  formed  before  the  salt  can  be  decomposed,  and 
not  till  then  will  the  chlorination  begin,  it  would 
seem  that  for  these  chemical  reactions  more  time  is 
required  than  a  few  seconds  ;  but  this  is  not  the 
case.  As  soon  as  ore  and  salt  enter  the  furnace, 
each  sulphuret  particle  ignites  in  the  glowing  at- 
mosphere, evolving  at  the  same  time  sulphur,  which 
in  presence  of  the  oxygen  of  the  atmospheric  air, 
coming  undecomposed  through  the  grates,  is 
turned  into  sulphurous  acid  and  the  metal  into  an 
oxide,  or  in  part  directly  info  a  chloride.  The 
sulphurous  acid  in  contact  with  the  ore  particles 
and  oxygen  becomes  sulphuric  acid.  The  temper- 
ature is  nearly  from  the  start  too  high  to  permit 
the  formation  of  sulphates,  so  that  the  sulphuric 
acid  turns  its  force  on  the  red  hot  salt  particles, 
setting  the  chlorine  free.  All  these  reactions  are 
performed,  instantaneously.  Steam,  emanating 
from  the  fuel,  is  also  amongst  the  gases,  conse- 
quently the  creation  of  hydrochloric  acid  must 
ensue.  The  whole  space  in  the  furnace  is  filled 
with  glowing  gases  of  chlorine,  hydrochloric  acid, 
sulphurous  and  sulphuric  acid,  oxygen,  steam,  vol- 
atile base  metal  chlorides,  etc. — all  of  them  acting, 
decomposing  and  composing,  on  the  sulphurets 
with  great  vigor.  The  chlorine  decomposes  the 


ROASTING    OF    ORES.  65 

sulpliurets  directly,  forming  chloride  of  metals  and 
chloride  of  sulphur  ;  it  attacks  decomposing^  also 
oxides  and  sulphates,  if  present.  The  hydrochloric 
acid  performs  the  same  office.  Also  metallic  silver, 
if  it  should  occur  in  the  ore,  would  combine  with 
the  chlorine.  The  sulphuric  acid,  besides  decom- 
posing the  salt,  oxidizes  partly  the  sulphurets  di- 
rectly, etc. 

Considering  now  an  ore  particle  in  a  red  hot 
condition  attacked  simultaneously  by  all  these 
gases  while  falling,  the  final  chloridizing  result  is 
inevitable.  The  finer  the  ore  particles  are,  the 
more  perfect  the  chlorination  ;  but  even  if  some 
coarser  parts  (to  a  certain  degree)  should  reach  the 
bottom  not  thoroughly  chloridized,  this  would  be 
finished  in  the  pile,  as  the  chlorination  and  evolu- 
tion of  chlorine  gas  continues  in  the  red  hot  accu- 
mulation on  the  bottom  of  the  furnace. 

38.  Chloridizing  Roasting  of  Silver  Ore  contain- 
ing Gold.  Generally  the  ore  containing  silver  and 
gold  is  roasted  with  salt,  converting  thereby  the 
silver  into  a  chloride,  while  the  gold  remains  in  a 
metallic  condition.  This  mode  of  roasting  is  quite 
satisfactory  for  the  subsequent  amalgamation  in 
pans.  But  if  those  metals  are  intended  to  be  ex- 
tracted by  a  solving  process,  where  no  amalga- 
mation takes  place,  the  gold  also  must  be  converted 
into  a  chloride  while  roasting.  By  roasting  ores  in 
which  gold  and  silver  is  present  with  salt,  chlo- 
ride of  gold  is  formed,  according  to  Plattner  ;  but 


66  ROASTING    OF    ORES. 

before  the  ore  becomes  red  hot,  the  gold  loses  a 
part  of  its  chlorine,  is  reduced  to  a  sub-chloride, 
and,  at  a  little  higher  degree  of  heat,  to  metallic 
gold. 

To  form,  chloride  of  gold  by  way  of  roasting,  a 
better  result  is  obtained  in  the  furnace  if  the  ore  is 
roasted  first  without  salt  till  the  smell  of  sulphur 
is  no  longer  perceptible  ;  and  then,  after  it  has 
cooled  down  to  a  low  temperature,  the  salt  is  added 
and  the  whole  stirred  for  some  time.  A  suitable 
form  for  a  furnace  would  be  a  long  hearth  furnace 
(§  49),  altered  in  such  a  way  that  the  second  hearth 
should  be  ten  or  twelve  inches  below  the  first ;  the 
arch,  however,  should  continue  in  a  straight  line. 
By  this  means  the  space  is  widened  and  the  tem- 
perature brought  down  to  the  proper  degree.  The 
ore  is  charged  on  the  first  hearth  near  the  bridge, 
and  roasted  in  the  usual  way,  oxidizing  the  sul- 
phurets.  When  this  is  effected,  the  ore  is  shifted 
over  in  the  lower  furnace,  and  the  upper  charged 
again.  As  soon  as  the  roasted  ore  assumes  a  dark 
red  heat,  the  salt  is  introduced  and  raked  for  two 
or  three  hours.  According  to  Roeszner,  a  combi- 
nation of  gold  oxide  of  soda  and  chloride  of  sodium 
(Au2  O3  Na  Cl)  is  formed.  It  is  not  soluble  in 
water,  and  but  slightly  in  a  salt  solution,  and  can- 
not be  amalgamated,  but  it  is  soluble  in  hyposul- 
phite of  soda  or  lime.  V.  Lill  and  others  consider 
the  gold  in  the  state  of  a  sub-chloride  (Au  Cl).  Hot 
water  cannot  be  used  for  the  purpose  of  leaching 
out  base  metals,  as  the  chloride  of  gold  would  be 
decomposed. 


ROASTING    OF    ORES.  67 


Proper  Roasting — Charges. 

39.  The  endeavor  to  perform  the  roasting  in 
the   most   economical  way,  leads  many  operators 
astray,  since  they  lose  sight  of  the  great   impor- 
tance of  the  chemical  reactions,  which,  as  the  main 
object,  have  to  be"  considered   first.     Everything 
has  its  limit,  and  so  the  quantity  of  ore  to  be  taken 
for  one  charge.     The  European  fashion  (charging 
from  four  to  five  hundred  pounds)  could  not  be 
well  adopted  in  the  United  States,  while  to  place 
2,000  pounds  at  once  in  a  furnace  might  do  for  a 
mere  desulphurization,  but  is  decidedly  too  much 
for  both  the  chloridizing  and  the  oxidizing  roasting 
of  silver  ores,  if  good  results,  by  one  or  the  other 
mode   of   extraction,  are  to  be  obtained.     Eight 
hundred  pounds,  or  at  least  not  over  1,000  pounds, 
is  a  permissible  quantity  in  a  properly  constructed 
furnace,  and  with  careful  handling. 

B. — Oxidizing   Roasting. 

40.  The  purpose  of  the  oxidizing  roasting  is 
either  to  expel  volatile  substances  which  are  com- 
bined with  the  metals  (as  sulphur  or  arsenic),  or  to 
expel   volatile   metals   which   are    considered   ob- 
noxious to  further  treatment  of  silver  ores  (as  anti- 
mony, lead,  zinc,  etc).     The   oxygen  has  a  large 
share  in  this  transaction,  and  combines  with  the 
volatile  substances,  as  well  as  with  the   metals. 


68  ROASTING    OF    ORES. 

Some  of  the  combinations  with  the  ox}rgen  become 
volatile  —  as,  for  instance,  sulphurous,  arsenous 
and  antirnonious  acids,  lead  and  zinc  oxides,  etc. 
Other  combinations  again  are  not  volatile,  as  the 
formed  sulphates,  arsenates  and  antiinonates. 
Some  of  these  latter  compounds  can  be  disengaged 
by  an  increased  heat,  as  the  sulphates  of  iron  and 
of  copper,  whereby  the  sulphuric  acid  escapes, 
while  the  remaining  metal  turns  into  an  oxide. 
Others  cannot  be  decomposed  by  an  increased  heat, 
or  an  increased  heat  is  considered  injurious  for 
other  reasons  ;  and  in  this  case  such  combinations 
may  be  decomposed  by  an  addition  of  charcoal 
powder,  saw-dust,  or  the  application  of  hydrogen. 
The  sulphuric  acid  is  reduced  by  the  carbon  to 
sulphurous  acid,  and  goes  off,  and  so  also  the 
arsenic  and  antimony.  The  carbon  deprives  the 
sulphate  or  arsenate  of  a  part  of  its  oxygen,  and 
escapes  as  carbonic  acid. 

Changes  of  the  most  common  Metals  while 
Roasting. 

41.  Iron  pyrites  and  copper  sulphurets  suffer 
different  changes,  based  on  the  action  and  reaction 
of  the  oxygen,  sulphurous  acid,  sub-oxides  and  sul- 
phurets,  before  a  complete  change  is  effected  into 
sulphates  of  iron  and  copper.  Raising  the  heat, 
the  sulphuric  acid  is  driven  out,  and  iron  oxide 
and  copper  oxide  remain  unchanged.  If  salt  is 
present,  in  place  of  the  oxides  first,  chlorides  are 


ROASTING    OF    ORES.  69 

formed,  whereby  both  became  volatile.  Galena,  or 
sulphuret  of  lead,  at  a  low  temperature,  turns  by  de- 
grees into  oxide  and  sulphate  of  lead,  the  first 
being  volatile.  At  a  higher  heat  the  sulphate  re- 
mains unchanged  and  cannot  be  decomposed  into 
an  oxide.  In  presence  of  salt  the  greatest  part  of 
the  lead  becomes  a  chloride,  which  is  volatile,  but  a 
part  of  it  loses  some  chlorine,  and  thus  being  re- 
duced to  a  basic  chloride,  is  no  longer  volatile. 
Antimony  is  volatile  as  an  oxide,  but  there  is  also 
antimonic  acid  formed,  which  combines  with  other 
metal  oxides  to  antimonates,  which  are  not  volatile 
and  not  decomposed  by  increased  heat.  As  a  chlo- 
ride the  antimony  is  very  volatile.  Zincblende,  or 
sulphur  el  of  zinc,  requires  a  long  roasting.  Oxide 
of  zinc  is  formed  besides  the  sulphate.  The  sul- 
phate of  zinc,  under  stronger  heat,  is  reduced  to  a 
basic  sulphate,  which  can  be  decomposed  to  an 
oxide,  but  only  at  a  white  heat.  As  a  chloride  it 
volatilizes. 

What  Process  Requires  Oxidizing  Roast- 
ing. 

/ 

42.  The  oxidizing  roasting  is  in  use  partly  as  a 
preparatory  treatment  for  a  chloridizing  roasting. 
It  is  independent  only  for  Ziervogel's  process  and 
for  the  chlorination  process  ;  that  is,  so  far  as  con- 
cerns the  extraction  of  silver  by  precipitation.  For 
amalgamation  of  silrer  ore  no  oxidizing  roasting  is 
suitable;  but  it  is  important  with  the  smelting  pro- 


70  ROASTING    OF    ORES. 

cesses,  and  also  in  extracting  gold  from  gold  ores 
—principally  from  sulphurets  (iron  pyrites).  For 
this  purpose  long  furnaces  (§  49)  are  the  most 
suitable. 

The  main  point  in  the  roasting  for  Ziervogel's 
process  is  the  creation  of  a  sulphate  of  silver,  and 
the  oxidation  of  the  base  metals  as  far  as  possible. 
As  the  decomposition  of  sulphates  of  different 
metals  depends  on  different  degrees  of  tempera- 
ture, such  roasting  appears  of  a  very  delicate  na- 
ture. To  this  process  principally  argentiferous 
copper  matt  is  subjected. 

43.  Boasting  of  Copper  Matt.  When  pulverized 
until  fine  enough  to  pass  through  a  sieve  t>f  thirty- 
three  holes  to  the  running  inch,  the  mass  is  intro- 
duced into  the  furnace  and  spread  out  by  means  of 
rakes.  The  matt  inclines  much  to  clotting.  For 
this  reason  a  very  moderate  temperature  is  applied, 
more  for  drying  than  for  roasting.  The  matt  is 
left -quiet  for  about  fifteen  minutes,  after  which  the 
stirring  is  commenced  and  continued  without  stop- 
ping for  an  hour.  During  this  time  many  lumps 
are  formed,  which  the  roaster  tries  to  crush  to 
powder.  Near  the  working  door  the  stuff  is  ex- 
posed to  a  draft  of  fresh  air,  in  consequence  of 
which  the  roasting  on  that  place  progresses  more 
rapidly  than  it  does  further  back.  This  makes  a 
shifting  of  the  stuff  necessary  after  one  hour's 
roasting.  The  other  roaster  now  takes  the  rake 
and  stirs  the  matt  again  for  an  hour,  doing  the 
work  precisely  as  the  first  roaster  did.  The  roast, 


ROASTING    OF    ORES.  71 

ers  change  in  this  way  every  hour  for  five  and  one- 
half  to  five  and  three-quarters  hours.  This  roasting 
is  performed  on  the  upper  hearth  of  the  double 
furnace.  Twenty -five  pounds  of  coal  dust  are 
mixed  with  the  matt,  causing  an  ignition  and 
emission  of  gases,  and  the  whole  mass  is  transferred 
to  the  lower  hearth  through  a  hole  in  the  bottom. 
The  upper  hearth  is  now  charged  with  500  pounds 
of  matt  anew. 

The  sulphur  commences  to  burn  after  a  raking  of 
three-quarters  of  an  hour,  and  the  mass  increases 
in  volume  when  the  hearth  is  covered  about  four 
inches  with  matt.  During  the  roasting  all  metals 
are  converted  into  sulphates,  of  which,  toward  the 
end  of  the  operation,  iron  and  zinc  vitriol  are  de- 
composed, leaving  those  metals  as  oxides.  Copper, 
nickel  and  cobalt  remain  in  the  rtate  of  sulphates. 

The  lower  hearth  is  in  a  light  red  hot  condition 
when  the  matt  falls  in  from  the  upper  hearth.  To 
prevent  the  rapid  burning  of  the  admixed  coal 
dust,  and  the  clotting  of  the  mass,  a  vigorous  stir- 
ring for  an  hour,  with  closed  dampers,  is  strictly 
observed.  The  stuff  is  now  shifted  and  then  the 
damper  opened.  There  now  follows  a  sharp  oxi- 
dizing roasting,  with  free  access  of  air,  for  one  hour 
and  a  half.  By  means  of  the  air  current,  the 
roasting  mass  is  cooled  down  so  far  that  it  appears 
quite  dark.  To  see  the  progress  of  the  roasting,  a 
sample  is  taken  out  and  examined,  either  on  a  porce- 
lain dish  or  on  a  filter  with  cold  water.  The 
leach  must  appear  of  a  clear  blue  color,  and  an 


72 


ROASTING    OF    ORES. 


addition  of  salt  solution  must  give  some  white  pre- 
cipitate, proving  the  beginning  of  the  formation  of 
sulphate  of  silver.  If  the  nitrate  shows  a  greenish- 
blue  color,  the  presence  of  sulphate  of  iron  is  ap- 
parent, and  in  this  case  the  oxidizing  roasting  must 
continue. 

The  purpose  of  the  addition  of  coal  dust  is  the 
reduction  of  sulphates  to  basic  salts,  whereby  sul- 
phurous acid  is  emitted.  With  the  opening  of 
the  damper  the  oxidation  progresses,  the  sulphate 
of  iron  is  decomposed  almost  entirely,  the  sub-oxide 
of  copper  turns  into  oxide,  and  when  the  oxidizing 
roasting  is  finished,  the  mass  contains  mostly 
oxides,  but  also  basic  salts.  There  are  copper, 
iron  and  zinc  oxides,  sulphates  of  copper  and  zinc, 
while  the  silver  as  yet  consists  principally  of  an 
undecornposed  sulphide.  The  next  stage  of  roast- 
ing at  an  increased  temperature  is  the  last  one.  It 
is  directed  toward  the  sulplmrization  of  the  silver 
and  complete  oxidation  of  the  base  metals.  It 
takes  two  hours  and  a  half  to  accomplish  this  re- 
sult, under  continuous  raking  and  increasing  the 
temperature  to  a  light  red  heat.  Samples  are 
taken  again  as  before,  and  examined  in  the  same 
way.  The  leach  must  appear  only  of  a  bluish  tint, 
and  on  adding  salt  solution,  a  heavy  precipitate 
must  fall,  caused  by  chloride  of  silver.  The  whole 
roasting  period  on  the  lower  hearth,  as  on  the 
upper,  takes  from  five  and  one-half  to  five  and 
three-quarters  hours. 

The  formation  of  the  sulphate  of  silver  in  the  last 


ROASTING   OF    ORES.  73 

period  at  an  increased  heat,  is  due  to  the  sulphuric 
acid  in  gaseous  form,  emanating  from  the  sulphate 
of  copper.  It  attacks  the  sulphide  of  silver,  and 
combines  with  it  to  a  sulphate.  Ninety-two  per 
cent,  of  the  silver  is  extracted  after  this  roasting. 
If  in  the  last  period  the  feeding  with  fuel  should 
be  carelessly  performed,  so  as  to  give  a  smoky 
flame,  some  copper  oxide  will  be  reduced  to  sub- 
oxide,  and  this  will  precipitate  metallic  silver  while 
leaching,  causing  a  loss.  If  the  roasting  should 
not  continue  long  enough,  some  sulphide  remains 
undecomposed  ;  and,  on  the  other  hand,  if  the 
roasting  should  last  too  long,  a  part  of  the  sulphate 
of  silver  would  be  decomposed  to  metallic  silver 
and  could  not  be  leached  out.  These  circum- 
stances show  that  this  kind  of  roasting  demands  a 
great  deal  of  attention,  in  order  to  obtain  a  perfect 
result.  The  temperature  on  the  lower  hearth  in 
the  beginning  is  500  to  550  degrees  Centigrade  ;  it 
sinks  then  to  425,  and  rises  again  at  the  end  of 
the  operation  to  770  degrees . 

44.  Boasting  of  Gold  Ores.  The  gold  is  gen- 
erally found  in  a  free  state  as  metallic  gold.  In 
this  state  it  is  easily  extracted  by  proper  amalga- 
mation. Often,  however,  the  gold  is  combined 
with  other  substances,  so  that  amalgamation  is  of 
no  avail  unless  the  gold  is  set  free  by  roasting. 
Iron  pyrites  and  arsenical  pyrites  are  the  principal 
ores  containing  the  gold  in  a  condition  unfit  for 
direct  amalgamation.  Also  telluride  of  gold  must 
4 


74  ROASTING    OF    ORES. 

be  subjected  to  roasting  before  amalgamated  or 
chlorinated,  but  this  mineral  is  not  often  found. 

The  roasting  of  sulphurets  and  arseniurets  is 
very  simple,  all  that  is  required  being  a  perfect, 
dead  roasting  ;  that  is,  expulsion  of  all  sulphur 
and  arsenic  ;  but  this  process  takes  generally  more 
time  than  a  chloridizing  roasting.  After  the  fur- 
nace has  been  heated  for  some  hours,  the  sulphu- 
rets are  introduced  into  it  and  spread  over  the 
roasting  hearth,  which  is  generally  about  twelve 
feet  by  twelve,  and  is  capable  of  receiving  one  ton. 
One  man  is  sufficient  to  attend  a  single  furnace, 
but  a  long  one  requires  two  men.  A  single  furnace 
commences  with  a  low  heat,  sufficient  to  start  the 
self -burning  of  the  sulphurets,  by  which  so  much 
heat  is  created  as  for  several  hours  to  require  but 
very  little  fuel.  Nearly  half  of  the  sulphur  is  ex- 
pelled with  this  low  heat.  On  exposing  a  fresh 
surface  of  the  mass  by  stirring,  the  burning  of  the 
sulphur  with  a  bluish  flame  can  be  seen  distinctly. 
The  hoe  is  principally  used .  for  stirring.  It  must 
be  as  light  as  possible,  seven  to  eight  feet  long,  if 
prepared  to  work  from  both  sides  of  the  furnace. 
The  stirring  is  performed  at  intervals  of  ten  to  fif- 
teen minutes,  but  not  longer  ;  and  wherever  the 
circumstances  permit  two  roasters  to  be  employed, 
the  time  of  roasting  will  be  shortened.  Oxidizing 
roasting  requires  more  stirring  than  the  chlo- 
ridizing. 

In  proportion  as  the  oxidation  of  the  sulphurets 
draws  nearer  to  the  end,  the  temperature  decreases, 


ROASTING    OF    ORES.  75 

and  it  is  then  necessary  to  use  more  fuel  to  keep 
the  mass  at  a  good  red  heat.  It  takes  from  twenty 
to  forty  hours  before  the  roasting  of  one  charge  in 
a  single  furnace  may  be  considered  finished.  If, 
in  throwing  up  sulphurets  in  the  furnace,  by  means 
of  a  shovel  or  hoe,  many  brilliant  sparks  appear, 
this  denotes  that  the  roasting  is  not  finished,  but 
must  be  continued  till  this  appearance  ceases. 

In  a  long  furnace  (Figs.  7,  8,  §  49)  the  hearth 
near  the  bridge  is  always  kept  at  a  bright  heat. 
One  man  attends  to  the  ore  on  the  first  hearth,  and 
thje  other  two  or  four  hearths  can  be  managed  by 
a  second.  In  moving  the  ore  from  one  hearth  to 
the  other,  or  in  drawing  the  charge  from  the'  fin- 
ishing hearth,  these  two  men  assist  each  other. 
The  finishing  hearth  receives  the  ore  already  desul- 
phurized to  a  great  extent,  and  containing  only  a 
small  part  of  undecomposed  sulphurets,  but  more 
of  sulphates.  With  a  lively  heat  and  active  stirring 
at  intervals,  all  base  metals  ought  to  be  converted 
into  oxides  after  ten  or  twelve  hours. 

An  addition  of  from  thirty  to  fifty  pounds  of  salt 
to  the  ton  of  sulphurets  at  the  end  of  roasting,  two 
or  three  hours  before  the  discharge,  is  not  injurious 
to  the  subsequent  chlorination,  but  it  increases 
uselessly  the  expense  if  mixed  with  such  sulphurets, 
when  there  is  no  necessity  for  it.  In  many  in- 
stances, however,  especially  where  the  sulphurets 
contain  lime,  calcspar,  talc  or  heavy  spar,  a  chlo- 
ridizing  roasting  is  necessary,  if  it  is  intended  to 
extract  the  gold  by  chlorination.  One  hundred 


76  ROASTING    OF    OEES. 

pounds  of  salt  are  sometimes  required  for  one  ton 
of  sulphurets. 

Sulphurets  containing  gold  can  be  brought  into 
a  soluble  condition  by  means  of  roasting,  accord- 
ing to  §  38,  so  that  no  chlorination  is  required 
after  roasting. 

Roasting   Furnaces. 

45.  Roasting  not  only  requires  much  care,  but 
it  is  also  an  expensive  operation.  For  this  reason 
the  choice  of  the  right  kind  of  furnaces  is  of  very 
great  importance,  and  so  much  the  more  as  a  per- 
fect and  economical  extraction  of  silver  depends 
principally  on  the  result  of  roasting.  The  chlori- 
dizing  roasting  is  known  to  be  the  most  suitable 
way  for  the  subsequent  extraction  of  silver  in  what- 
ever way  it  may  be  performed,  by  amalgamation  or 
solving  ;  consequently  those  furnaces  in  which  the 
ore  particles  are  exposed  to  the  action  of  chlorine 
and  other  chloridizing  gases  to  the  most  advantage, 
must  be  considered  the  best.  The  old  style  of 
furnace  was  four  to  six  feet  wide  and  ten  feet  long, 
and  in  them  a  small  part  of  the  ore  was  exposed  to 
the  greatest  heat  near  the  bridge.  The  gases 
evolved  were  carried  along  by  the  draft,  being  in 
contact  with  the  surface  of  the  ore  for  a  length  of 
ten  feet  while  passing  over  it  ;  but  on  account  of 
the  narrowness  of  the  hearth,  the  ore  at  the  bridge 
had  to  be  changed  often  with  the  cooler  part  at  the 
flue. 


ROASTING   OF    ORES.  77 

The  next  step  in  improvement  was  the  adoption 
of  wicler  hearths,  even  wider  than  long.  The  heat 
was  more  uniform  and  the  result  better.  In  both 
kinds  of  furnaces  the  chlorination  of  the  metals  de- 
pends principally  on  the  chlorine  developed  in  the 
mass  of  the  ore  while  passing  through  it ;  but  once 
above  the  surface,  the  chlorine  and  volatile  chlo- 
ride metals  have  little  chance  to  transmit  their 
chlorine  to  the  ore  (§  23),  and  this  only  through 
the  chlorination  period.  During  two  or  three  hours 
of  each  charge,  when  desulphurization  and  sul- 
phatization  are  going  on,  this  must  be  performed 
by  the  oxygen  of  the  air,  while,  if  chlorides  were 
present  from  the  beginning,  sulphurets,  sulphates 
and  oxides  would  have  been  partly  decomposed  di- 
rectly by  the  chlorine,  whereby  time  and  a  certain 
percentage  of  salt  are  saved. 

In  this  respect  a  great  advantage  is  gained  by 
the  introduction  of  "long  furnaces"  (§  49),  in 
which  a  continual  formation  of  chlorides  on  the 
finishing  hearth  near  the  bridge  is  going  on,  vola- 
tile chlorides  and  free  chlorine  being  evolved, 
which,  on  their  way  to  the  flue,  are  constantly  in 
contact  with  the  ore  for  a  space  of  thirty  or  fifty 
feet  in  length.  These  furnaces  show  a  great 
economy  in  fuel,  labor  and  salt,  and  the  roasted 
ore  contains  a  better  percentage  of  chloride  of 
silver  (§  36). 

Another  most  important  improvement  in  the  way 
of  chloridizing  roasting  is  found  in  the  Stetefeldt 
furnace  (§  58),  where  all  ore  particles  are  involved 


78  ROASTING    OF    ORES. 

in  chloridizing  gases  under  very  favorable  circum- 
stances. The  roasting  is  cheap,  and  from  twenty 
to  twenty-five  tons  of  ore  are  roasted  in  twenty- 
four  hours — more  than  ever  accomplished  in  any 
other  furnace. 

The  roasting  furnaces  do  not  require  a  white  heat; 
hence  common  bricks  can  be  used  ;  but  it  is  nev- 
ertheless advantageous  if  the  fire-place  above  the 
grates  is  built  of  fire  bricks.  In  new  or  unpopu- 
lated districts  even  unburned  bricks  or  adobe  may 
be  used  ;  they  stand  just  as  well  as  burned  bricks 
of  the  same  material,  except  on  the  floor  of  the  fur- 
nace, which  is  worked  out  in  two  or  three  months. 
Hard  bricks  are  the  best  material  for  the  hearth- 
floor,  placed  edgeways  (four-inch),  with  as  little 
clay  between  as  possible,  and  laid  carefully  and 
well  fitting,  so  as  to  form  a  level  and  smooth  sur- 
face. All  parts  exposed  to  heat  must  be  built  with 
loam  or  clay,  not  with  mortar.  Many  masons  have 
the  custom  of  laying  three  heights  of  bricks  so  that 
the  eight-inch  wall  is  formed  by  two  rows  length- 
ways, and  only  the  fourth  height  is  put  crossways. 
It  is  a  quick  work  and  may  answer  for  buildings, 
but  should  not  be  allowed  with  furnaces  where  the 
expansive  heat  must  be  considered,  especially  in 
the  fire-place.  Each  alternate  row  of  bricks  must 
be  laid  crossways  to  the  preceding  ;  also,  adjusting 
the  wall  with  the  hammer,  to  make  it  perpendicular 
and  square,  after  several  bricks  are  laid,  is  injurious. 
The  outside  appearance  of  a  furnace  is  of  minor  im- 
portance, and  the  mason  must,  contrary  to  his  gen- 


BOASTING    OF    ORES.  79 

eral  idea,  pay  the  most  attention  to  the  solid  and  par- 
ticular work  inside.  The  distance  of  the  arch  from 
the  hearth  is  from  eighteen  to  twenty  inches  in  the 
highest  point,  not  far  from  the  bridge  ;  in  a  long 
furnace,  however,  the  roof  of  the  first  hearth  can 
be  higher  from  the  floor  by  four  to  five  inches,  ac- 
cording to  the  length.  An  eight-inch  thickness  of 
the  arch  is  sufficient,  and  the  bricks  laid  with  the 
eight-inch  side  perpendicular  form  a  more  dura- 
ble arch  than  one  of  twelve  inch  thickness  com- 
posed of  eight  and  four-inch  sides  of  the  bricks. 
The  furnace  must  be  secured  against  expansion  by 
grappling-irons  of  cast  iron  tightened  with  iron 
rods  from  five  to  six-eighths  of  an  inch  in  diameter. 
The  rods  placed  over  the  length  of  the  furnace  are 
stronger — one  inch  in  diameter.  In  place  of  iron 
grapplings,  also  wooden  posts,  six  by  eight  inches, 
are  used,  tied  by  iron  rods  on  the  top.  The  lower 
ends  are  generally  put  in  the  ground,  but  it  is 
preferable  to  use  rods  on  both  ends.  In  case  of 
need,  even  the  rods  are  replaced  by  timber.  For 
the  passage  of  the  rods  square  holes  must  be  pro- 
vided, in  the  masonry;  also  for  the  escape  of 
dampness  such  passages  are  necessary  at  different 
points,  especially  if  the  whole  block  consists  of 
masonry.  The  floor  of  the  hearth  should  be  three 
feet  and  a  half  above  the  ground  ;  if  lower,  it  is 
inconvenient  for  the  roaster. 

There  are  two  principal  classes  of  furnaces — such 
as  are  managed  by  hand  and  such  as  employ  ma- 
chinery. For  the  first  class  mostly  reverberatory 


80  ROASTING    OF    ORES. 

furnaces  are  in  use.  The  second  class  comprises 
reverberatory,  cylindrical  and  vertical  furnaces. 

A.     Roasting   Furnaces  Managed   by 
Handwork. 

46.  Beverberatory    Furnaces.      Reyerberatory 

furnace  is  the  name  applied  to  all  horizontal  hearth 
furnaces  provided  with  grates  and  fire-place  on  one 
side,  and  a  flue  connected  with  a  chimney  on  the 
other.  The  draft  here  is  created  by  the  chimney 
instead  of  by  bellows,  as  in  blast  furnaces  ;  there- 
fore only  such  fuel  is  used  which  gives  a  flame,  and 
consequently  no  charcoal,  coke  or  anthracite  is 
serviceable  unless  in  a  gas  reverberatory  furnace, 
where  gas  (carbonic  oxide)  is  produced  from  char- 
coal or  other  fuel — sometimes  alsa  by  the  aid  of 
compressed  air — and  burned.  The  reverberatory 
roasting  furnaces  are  constructed  in  various  ways. 
There  are  single  furnaces,  with  but  one  hearth,  and 
double  furnaces,  with  two  hearths,  one  above  the 
other.  Sometimes  above  the  second  hearth  there 
is  a  third  one  for  the  purpose  of  drying  the  charge. 
Long  furnaces  also  are  coming  into  use. 

47.  A  Single  Roasting  Furnace  is  represented  by 
Fig.  3,  showing  the  section,  and  Fig.  4,  the  ground 
plan.     The  bottom,  a,  or  the  hearth,  is  made  of 
the  hardest  bricks,  laid  edgewise  and  as  close  as 
possible.     Some  masons  lay  the  bricks  flat.     This 
mode  is  cheaper  and  quicker,  but  far  inferior  and 


ROASTING    OF    ORES. 


81 


less  durable  than  the  former  way,  and  requires  a 
more   carefully    prepared   foundation.     The   very 

Fig.  3. 


2345 


best  bricks  must  be  selected  for  the   hearth,     b 
4* 


82  ROASTING    OF    ORES. 

shows  the  discharge  hole  in  front  of  the  hearth. 
It  is  more  convenient  to  draw  the  ore  toward  the 
front  hole  than  to  have  a  door  for  this  purpose  be- 
hind, but  circumstances  may  decide  fo*  such  dis- 
charge doors.  The  flue,  e,  is  in  connection  with 
the  flue-holes,  ef,  in  the  arch,  as  indicated  by  dotted 
lines  in  Fig.  4,  and  is  from  nine  to  ten  inches  in 
diameter.  The  flue-holes  in  the  arch  have  the  ad- 
vantage that  no  ore  can  enter  when  being  stirred, 
as  often  happens  when  the  flue  commences  at  the 
hearth.  The  distance  between  arch  and  hearth 
near  the  bridge  is  twenty  to  twenty-one  inches,  and 
near  the  flue  only  eight  inches.  The  flue  leads 
into  the  chimney  in  any  suitable  direction,  either 
directly  or  through  a  dust  chamber.  Often  the 
flue  is  led  under  the  floor  (when  the  chimney  is  at 
some  distance  from  the  furnaces),  and  is  made 
wide  enough  to  serve  as  a  dust  chamber — say  two 
feet  wide  and  three  feet  high,  or  wider  if  several 
furnaces  are  connected  therewith.  The  chimney 
is  from  twenty  to  fifty  feet  high,  and  from  one  and 
one-half  to  three  or  four  feet  square  in  the  clear. 
On  the  top  of  the  chimney  an  iron  cover,  controlled 
by  a  chain,  regulates  the  draught.  This  is  prac- 
ticable only  when  but  one  furnace  is  attached  to 
the  chimney,  otherwise  dampers  must  be  provided 
for  each  furnace  in  the  flue.  The  bridge,  i,  is 
much  exposed  to  injury  by  fire  on  one  side,  and  by 
raking  on  the  other ;  it  is  therefore  advantageous 
if  the  upper  part,  or  the  whole  bridge,  can  be  made 
in  two  or  three  parts  and  of  some  fire-proof  stone, 


ROASTING    OF    ORES.  83 

— sandstone,  granite,  or  some  conglomerate,  which 
does  not  burst  when  heated.  The  grates,  h,  are 
twelve  to  sixteen  inches  below  the  top  of  the  bridge, 
eighteen  inches  wide,  and  from  six  to  seven  feet 
long.  The  space  between  the  grate-bars  is  one- 
fourth  to  one-half  of  an  inch. 

In  the  roof,  nearer  to  the  bridge,  is  an  opening 
four  to  five  inches  square,  of  cast  iron,  in  con- 
nection with  a  funnel,  I,  of  sheet  iron.  This  fun- 
nel must  be  large  enough  to  receive  one  charge  of 
the  ore.  A  slide  keeps  the  ore  in  the  funnel. 
The  roof  must  be  either  eight  inches  thick,  or  the 
double  length  of  brick  ;  that  is,  sixteen  inches. 
Under  the  hearth  there  is  an  arched  space,  d,  into 
which  the  roasted  ore  is  drawn  through  the  dis- 
charg'e  hole,  6,  either  directly  into  an  iron  car  or  on 
an  inclined  floor,  on  which  the  ore  slides  from  un- 
derneath the  furnace.  In  front  this  space  is  shut 
up  by  brickwork.  For  the  purpose  of  easy  drying- 
it  is  well  to  leave  open  some  holes,  g,  for  the  escape 
of  dampness.  It  is  not  necessary  to  build  the 
block  under  the  hearth  solidly  of  bricks.  The 
space  inside  is  generally  filled  up  with  rubbish  of 
bricks  and  stone. 

The  working  door,  o,  is  from  twenty-five  to  thirty 
inches  wide.  In  front  of  it  is  an  iron  roller  for 
easier  handling  of  the  heavy  tools.  The  door  is 
eight  to  nine  inches  high.  The  cast  iron  door- 
frame, p,  for  the  fire-place,  is  from  nine  to  twelve 
inches  square.  When  completed,  the  furnace  is 
tied  by  iron  rods,  n,  both  ways.  The  uprights 


84 


ROASTING    OF    ORES. 


are  generally  wooden  ones,  six  by  six  or  five  by 
eight  inches. 

It  is  very  important  to  dry  the  furnaces,  when 
finished,  with  a  very  moderate  fire  for  five  or  six  days, 
day  and  night.  Upon  a  slow,  gradual  drying,  the 
durability  of  the  arch  depends.  The  furnace  must 
be  nearly  red  hot  before  the  first  charge  of  ore  is 
introduced. 

48.     A  Double  Boasting  Furnace  is  represented 
Fig.  5. 


in  Fig.  5,  in  longitudinal  cross  section.     The  lower 
hearth,  a,  is  nine  feet  long  and  ten  feet  wide.    The 


OF  THB 

UNIVERSITY 


BOASTING   OF     ORES. 


roof  in  the  center  is  eighteen  inches,  and  at  the 
flue  and  bridge  fourteen  inches  above  the  hearth. 
The  fire-place,  r,  is  twenty  inches  wide,  eight  feet 
long,  and  twenty  inches  from  the  roof.  The  flue, 
6,  ascends  to  the  upper  hearth,  c,  the  working  door, 
o,  of  which  is  on  the  back  side.  In  case  there  should 
be  required  more  heat  than  is  obtainable  from 
the  lower  hearth,  there  is  an  auxiliary  fire-place,  r' . 
The  flame  goes  through  the  flue,  &',  into  the  dust 
chamber,  g.  This  chamber  has  four  cross  par- 
titions lengthways,  by  which  the  draught  is  forced 
to  take  a  longer  way  before  it  enters  the  chimney. 
From  the  upper  hearth  the  ore  is  drawn  through 
the  hole-,  d,  to  the  lower  hearth,  as  the  bridge  does 
not  permit  the  flue  to  be  used  for  this  purpose. 
e,  e,  are  canals  for  the  escape  of  moisture.  The  two 
hearths  can  be  used  separately  if  needed.  In  this 
case  the  flue,  b,  is  closed  and  another  one  (not  seen 
in  the  drawing)  opened.  This  second  flue  commu- 
nicates directly  with  the  dust  chamber. 

49.  Long  Roasting  Furnace.  This  kind  of 
roasting  furnace,  as  represented  by  Fig.  6  in  vertical 
section,  and  Fig.  7  in  ground  plan,  gives  much 
satisfaction,  as  there  is  not  only  a  great  saving  of 
fuel  effected,  but  also  a  greater  quantity  of  ore  can 
be  roasted  in  a  given  time  than  with  a  single  fur- 
nace. It  is  only  a  modification  of  the  double 
furnace,  but  it  seems  to  be  more  convenient  for 
the  roasters.  The  heat  is  better  utilized,  as  the  flame 
has  not  to  pass  through  flues  between  the  hearths, 


86 


ROASTING    OF    ORES. 


and  is  not  broken  so  often, 
but  the  moving  of  ore  from 
one  hearth  to  the  other  is 
more  troublesome.  There 
are  two  men  employed  at  a 
time,  there  being  one  ton 
and  a  half  to  two  tons  in 
the  furnace.  The  hearths 
are  either  arranged  horizon- 
tally, as  the  drawings  show, 
or  only  the  first  one  is  level; 
the  other  two  are  inclined  ; 
this  facilitates  the  shifting 
of  the  ore.  Each  hearth  is 
ten  feet  long  and  ten  or 
twelve  feet  wide.  After  the 
first  hearth  there  is  a  step  of 
four  to  six  inches,  partly  to 
divide  the  first  from  the  oth- 
ers, but  principally  to  con- 
tract the  space  between  roof 
and  hearth  of  the  other  two. 
The  ore  is  fed  on  the  last 
hearth  through  the  sheet 
iron  funnel,  a,  spread  equally 
on  &',  and,  according  to  its 
dampness  or  the  quantity  of 
sulphurets  contained,  stirred 
more  or  less  for  one  and  a 
half  to  two  hours.  As 
it  is  not  only  inconven- 


ROASTING    OF    ORES. 


87 


venient,  but  im- 
possible to  have  a 
good  stirring  ef- 
fected at  a  dis- 
tance of  twelve 
-feet,  which  re- 
quires long  and 
heavy  tools,  there 
are  for  this  reason 
working  doors  on 
both  sides  of  the 
furnace .  The 
roaster  uses  hoes 
or  rakes  eight  feet 
long,  made  partly 
of  gas  pipe,  which 
are  light  and  han- 
dy. The  working- 
doors  are  thirty 
inches  wide.  They 
must  all  be  kept 
closed  except  when 
the  ore  is  being 
raked,  and  then  it 
is  very  proper  to 
have  half  of  the 
door  closed  (with 
a  piece  of  sheet 
iron).  Sufficient 
air  comes  in  at  the 
Hl^  working  door  of 
the  first  hearth. 


88  ROASTING    OF    ORES. 

After  one  and  a  half  to  two  hours  the  ore  is  re- 
moved to  the  second  hearth,  from  b  to  c'  and  from 
b'  to  c,  and  again  spread  over  the  whole  of  c,  c' . 
Another  charge  is  introduced  onb,bf.  The  second 
hearth  has  a  better  heat  than  the  third  one.  The 
ore  is  treated  here  as  before,  being  raked  as  often 
as  possible.  After  a  lapse  of  one  and  a  half  to  two 
hours  the  ore  is  moved  again  to  the  first  hearth,  in 
the  same  way  as  before  ;  that  is,  from  c  to  d'  and 
from  c'  to  d.  The  ore  is  now  exposed  to  a  light 
red  heat,  by  which  the  chlorination  or  oxidation 
must  be  finished  in  the  same  time  as  on  the  other 
hearth.  It  is  necessary  to  change  here  the  ore 
from  the  bridge  toward  the  flue,  and  reverse  once 
during  the  roasting.  When  the  operation  is  fin- 
ished, the  roasted  ore  is  drawn  into  iron  cars  below 
the  furnace  through  the  opening,  e.  When  all  the 
ore  has  been  removed,  the  charge  on  the  second 
hearth  is  transferred  to  the  first,  from  the  third  to 
the  second,  and  from  the  funnel  to  the  third 
hearth,  and  the  process  continued  as  before,  so 
that  a  thousand  pounds  are  drawn  out  every  one 
and  a  half  or  every  two  hours. 

The  bridge,/,  is  fourteen  inches  high.  For  the 
purpose  of  admitting  air  or  steam,  a  canal  can  be 
made  in  it.  The  fire-place,  g,  is  eighteen  inches 
wide  and  eight  to  nine  feet  long,  and  fifteen  inches 
below  the  top  of  the  bridge.  The  ash-pit,  h,  is 
made  according  to  what  seems  more  convenient, 
as  represented  either  in  Fig.  6  or  in  Fig.  3.  A  deep 
ash-pit  is  more  favorable  for  the  preservation  of 


ROASTING   OF    ORES.  89 

the  grates,  as  they  are  less  heated.  Each  door  is 
provided  with  an  iron  roller,  i.  A  furnace  of  a 
similar  description  is  in  operation  in  La  Dura 
(Mexico),  roasting  refractory  silver  ores  for  the 
chlorination  process. 

A  furnace  sixty  feet  in  length,  with  six  hearths, 
as  built  by  Mr.  Graff  at  the  San  Marcial,  has  the 
advantage  of  being  capable  of  roasting  from  eight 
and  one-half  to  twelve  tons  of  ore  in  twenty-four 
hours,  discharging  every  hour  from  eight  hundred 
to  one  thousand  pounds,  according  to  the  charge. 
In  case  ore  is  subjected  to  roasting  which  has 
not  enough  sulphur  to  create  the  required  heat  in 
burning,  an  additional  smaller  fire-place  must  be 
attached  on  one  side,  so  as  to  bring  the  flame  into 
the  fourth  hearth. 

Muffle   Furnaces. 

50.  A  muffle  furnace,  as  the  name  indicates,  is 
a  furnace  constructed  of  clay  and  cast  iron  in  such 
a  way  as  to  prevent  the  flame  from  coming  inside 
of  it.  The  fuel  heats  the  mantle  or  muffle  from 
the  outside,  so  that  the  ore  is  not  heated  directly 
by  the  burning  fuel,  but  by  the  glowing  muffle. 
The  muffle  furnaces  require,  therefore,  more  fuel 
to  obtain  a  certain  degree  of  heat  than  ordinary  re- 
verberatory  furnaces,  where  the  flame  comes  into 
contact  directly  with  the  ore.  The  use  of  this  fur- 
nace is  limited,  and  applicable  in  cases  where  the 
air  or  the  gases  of  the  burning  fuel  are  injurious, 


90  ROASTING    OF    ORES. 

or  where  volatile  substances  from  the  ore  should  be 
condensed  ;  as  for  instance,  sulphur,  zinc,  arsenic, 
etc.  .For  roasting  silver  ores,  these  furnaces  are 
not  in  use,  but  they  were  tried  in  California  in  dif- 
ferent ways ;  also  for  desulphurization,  adding 
charcoal  to  the  pulverized  ore.  The  experiments, 
however,  were  not  successful,  as  could  have  been 
anticipated  (§  5,  e). 

B.    Roasting  Furnaces  with  Mechanical 
Arrangements. 

5 1 .  There  is  a  great  variety  of  furnaces  wherein 
the  costly  stirring  by  hand  is  replaced  by  mechan- 
ical apparatus.  No  mechanical  furnace  can  be 
governed  in  every  part  of  the  roasting  process  with 
the  same  facility  and  precision  as  is  possible  in  a 
reverberatory  furnace  with  manual  labor  ;  but  in 
the  latter  case  the  great  difficulty  in  finding  good 
reliable  roasters,  and  the  heavy  expenses  connected 
therewith,  make  a  mechanical  substitution  very  de- 
sirable. In  one  respect,  vertically  revolving  fur- 
naces, in  which  the  ore  moves  without  being 
stirred  by  mechanical  rakes  or  plows,  have  the  ad- 
vantage, viz  :  in  simplicity.  The  stirring  furnaces 
combine  sometimes  complicated  machinery  with  a 
general  defect,  and  this  is  the  wear  of  the  shoes  or 
plows,  not  on  the  lower  part  alone,  but  also  on  the 
sides.  Some  stirrers  are  fixed,  and  consequently 
cannot  remedy  the  wear  by  sinking  ;  but  none 
have  been  yet  so  constructed  as  to  keep  their 


ROASTING    OF    ORES.  91 

original  line  on  the  wall  side.  The  consequence 
is,  that  while  the  shoe  wears  away,  the  ore  takes 
its  place  and  hardens  there  (being  exposed  to  heat 
so  long)  till  new  shoes  are  put  in ';  and  in  this  case 
all  the  hard  cakes  are  broken  off  and  mixed  with 
the  ore.  But  as  new  shoes  are  not  put  in  every 
day,  these  periodical  lumps  may  be  sifted  and  re- 
turned to  the  battery.  The  accumulation  of  ore  in 
some  parts  of  the  furnace  successively,  cannot  be 
avoided  entirely,  even  in  furnaces  where  no  stirring 
goes  on. 

A.    Stirring  Furnaces. 

52.  Eevolving  Hearth  Furnace.  The  shape  of 
this  furnace  is  circular.  There  is  an  iron  frame  of 
from  ten  to  twelve  feet  diameter,  with  sides  four- 
teen inches  high.  The  whole  is  lined  with  brick, 
the  bottom  four  inches  thick.  The  discharge  open- 
ing is  on  the  bottom,  extending  from  the  periphery 
toward  the  center,  and  is  four  inches  wide  and 
three  feet  three  inches  long.  This  opening  is  shut 
by  an  iron  door,  hung  on  hinges.  It  is  not  neces- 
sary to  fill  this  space  with  brick,  which  would  in- 
terfere with  the  easy  opening  ;  but  the  space,  after 
the  discharge  of  the  ore,  must  be  filled  up  with 
roasted  ore,  of  which  enough  is  always  left  in  the 
furnace.  The  bottom  is  fixed  to  an  upright  shaft, 
four  inches  in  diameter,  provided  with  a  spurwheel 
at  the  lower  end  to  impart  the  motion.  This  ten 
or  twelve-foot  bottom  is  surrounded  by  a  substan- 


92  KOASTINQ    OF    ORES. 

tial  ring  wall,  as  close  to  the  periphery  of  the  bot- 
tom as  possible.  The  bottom  is  then  arched  over 
with  bricks,  leaving  the  doors  through  which  the 
new  shoes  are  introduced  when  the  old  ones  wear 
out.  There  is  also  a  cast  iron  pipe  through  the 
center  of  the  furnace,  on  which  the  shoes  are 
fastened  and  so  arranged  that  one  set  plows  the 
ore  against  the  center,  the  other  set  toward  the 
periphery.  The  pipe  is  hollow  and  cooled  by  a 
continual  stream  of  water.  There  is  also  a  hole 
four  to  five  inches  square  in  the  arch,  in  connection 
with  a  funnel,  through  which  the  ore  is  charged 
into  the  furnace.  The  distance  from  the  bottom 
to  the  center  of  the  arch  is  thirty-one  inches.  The 
arch  is  connected  on  one  side  with  the  fire-place, 
six  or  seven  feet  long  and  eighteen  or.twenty  inches 
wide,  and  about  ten  inches  below  the  rim  of  the 
revolving  hearth  are  the  grates.  On  the  opposite 
side  is  the  connection  with  the  flue. 

Such  furnaces  have  the  advantage  that  they  carry 
the  ore  in  a  circle,  so  that  each  part  is  equally  ex- 
posed to  the  heat  near  the  bridge  and  to  the  cooler 
region  near  the  flue.  While  revolving,  the  funnel 
is  opened  and  the  ore  falls  on  the  moving  bottom, 
being  spread  in  passing  under  the  stationary  stir- 
rers,  which  are  of  a  plow  shape.  The  roasting 
takes  about  the  same  time  as  in  an  ordinary  fur- 
nace, but  requires  less  fuel,  as  the  furnace  is  not 
cooled  down  by  air,  which  enters  the  common  re- 
verberatory  furnace  through  the  working  door.  It 
is  important  to  have  the  horizontal  shaft  provided 


ROASTING    OF    OEES.  93 

with  two  driving  wheels  of  different  size,  so  that 
about  one  to  two  revolutions  per  minute  can  be  ob- 
tained while  roasting,  and  from  six  to  eight  revo- 
lutions while  discharging.  After  the  roasting  is 
finished,  the  discharge  door  on  the  bottom  can  be 
opened,  while  the  hearth  revolves  slowly.  In  this 
furnace  it  is  an  easy  matter  to  arrange  the  plows 
in  such  a  way  that  they  could  be  moved  every 
second  or  third  day  toward  tho  periphery  as  much 
as  they  wear  off.  In  this  way  the  side  of  the 
hearth  can  be  kept  always  clear  from  accumu- 
lation of  the  ore  crust. 

53.  A  similar  furnace  is  Brunton's  revolving 
furnace.     The  hearth  has  a  low  conical  shape,  the 
highest  point  being  the  center.     Above  this  is  the 
charging  hole  in  the  roof.     The  hearth  is  twelve 
feet  in  diameter,  and  takes  one  ton  of  tin  ore  at  a 
charge.     There  is  a  cast  iron  rake  with  three-inch 
long  prismatic  teeth,   which   are   dovetailed  and 
so  constructed  as  to  be  easily  replaced.     The  ore 
comes  through  a  funnel  in  the  center  of  the  revolv- 
ing hearth,  and  is  spread  by  the  stationary  rake, 
the  position  of  which  is  not  radial  but  oblique. 
The  hearth  is  fixed  to  a  solid  vertical  shaft  with 
gearing,  by  which  a  slow  rotating  motion  is  im- 
parted to  the  hearth,  so  that  only  one  revolution  is 
made  in  forty  minutes. 

54.  Ernst's  Rotary  Furnace  is  constructed  on 
the  same  principle  as  the  former  two,  but  differs 


94  ROASTING   OF    ORES. 

materially  in  two  points.  While  in  the  before  de- 
scribed revolving  hearth  furnaces,  the  diameter  can- 
not be  increased  well  beyond  twelve  or  fifteen  feet, 
principally  for  the  reason  that  the  roof  would  stand 
too  far  off  from  the  bottom,  Ernst's  furnace  can 
be  constructed  on  any  given  diameter.  The  other 
point  of  difference  lies  in  the  discharge,  which  is 
continuous  with  Ernst's  furnace. 

The  hearth  is  a  circular  iron  ring  (disk),  lined 
with  bricks,  and  revolving  on  a  series  of  iron  roll- 
ers. It  is  kept  in  motion  "fey  means  of  two  gear 
wheels,  each  three  feet  in  diameter.  The  speed  is 
regulated  by  a  cone  pulley,  being  increased  or  di- 
minished to  conform  to  the  require^  motion  of  the 
hearth.  The  ore  is  charged  continuously  from  the 
battery,  through  a  hopper,  by  means  of  elevators. 
The  hearth  moves  constantly,  carrying  the  ore  from 
the  flue  toward  the  fire-place,  and  exposing  it  to 
all  the  different  temperatures  of  the  furnace,  thus 
effecting  a  very  uniform  heating  of  the  ore.  The 
hearth-ring  is  surrounded  by  a  corresponding  wall 
of  masonry,  leaving  a  free  circular  space  in  the 
center,  where  the  driving  machinery  is  placed. 
There  are  stationary  and  movable  stirrers.  The 
movable  paddle-stirrers  are  connected  with  the 
gearing  under  the  iron  bed  of  the  hearth  on  its  in- 
side periphery.  The  ore  finally  arrives  at  the  fire- 
bridge, where  it  receives  its  ultimate  and  highest 
heat,  and  is  then  discharged  by  an  apparatus, 
which  consists  of  two  chain  or  rag  wheels,  by 
which  a  double  endless  chain,  with  inserted  plates 


ROASTING    OF    ORES.  95 

of  iron,  is  carried  across  the  hearth,  thus  discharg- 
ing the  roasted  ore. 

This  furnace,  not  as  yet  in  use,  promises  to  do 
good  work.  In  some  respects  it  could  be  com- 
pared with  O'Hara's  furnace — having  a  long,  nar- 
row hearth  ;  replacing  the  endless  chain,  however, 
by  the  motion  of  the  hearth,  both  being  self -dis- 
charging. For  chloridizing  roasting  of  silver  ore, 
the  center  circle  of  the  ring  must  have  a  diameter 
of  about  twenty-six  feet.  One  revolution  should 
be  accomplished  in  five  or  six  hours.  It  takes 
about  twenty  stirrers  to  rake  the  ore  every  ten 
minutes,  if  equally  divided.  It  is,  however,  more 
proper  to  arrange  the  stirrers  closer  toward  the 
feeding  place. 

55.  Parke's  Roasting  Furnace,  with  movable 
stirrers.  This  is  a  double  furnace,  one  hearth 
above  the  other,  with  a  common  vertical  shaft  to 
which  the  stirrers  are  fastened.  The  hearth  is 
twelve  feet  in  diameter  and  rests  on  an  arch,  be- 
neath which  the  rotating  motion  is  transferred  to 
the  shaft  by  means  of  gearing.  On  one  side  of  the 
lower  hearth  is  the  fire-place,  whence  the  flame 
draws  over  the  bridge  into  the  furnace. 

Opposite  the  bridge  is  an  opening  one  foot  wide 
and  four  feet  long,  through  which  the  flame  ascends 
to  the  upper  hearth.  Both  of  the  hearths  have 
two  working  openings,  which  are  closed  by  cast- 
iron  doors.  From  the  upper  hearth  the  flame 
draws  through  a  flue  into  the  chimney.  The  shaft 


96  ROASTING    OF    ORES. 

goes  through  both  hearths  and  the  roof.  There 
are  two  massive  arms  in  both  furnaces,  with  curved 
spikes  attached  for  the  purpose  of  stirring  the  ore. 
In  order  to  keep  the  shaft  cool,  it  is  hollow,  and  a 
few  holes  above  the  gear  permit  the  cold  air  to 
draw  through  the  shaft,  whereby  a  constant  cool- 
ing is  effected.  The  upper  end  of  the  shaft  runs 
in  a  cast  iron  cross,  fixed  on  the  roof  of  the  furnace. 
After  the  ore  on  the  lower  hearth  is  drawn  out 
through  the  discharge  hole  at  the  bottom,  the  ore 
on  the  upper  hearth,  already  desulphurized  to  a 
great  extent,  is  raked  toward  a  similar  discharge 
hole,  and  then  transferred  to  the  lower  depart- 
ment. The  raw  ore  is  charged  through  the  roof 
into  the  upper  part.  By  means  of  hoes  the  ore 
is  spread  on  both  hearths,  before  the  shaft  is 
allowed  to  revolve  again. 

56.  Bruckner's  Revolving  Furnace.  An  iron 
cylinder  about  ten  feet  long  and  four  feet  in  diam- 
eter is  lined  with  bricks  inside.  The  cylinder  is 
fixed  with  its  long  axis  in  a  horizontal  position. 
One  end  communicates  with  a  fire-place,  from 
where  the  flame  passes  through  the  revolving  cyl- 
inder into  the  flue  at  the  other  end.  Inside  there 
is  an  inclined  shelf,  the  position  of  which  is  such 
that  the  ore  is  being  continually  shifted  from  one 
end  of  the  cylinder  to  the  other,  as  this  revolves 
and  thereby  thoroughly  exposed  to  the  flames.  In 
this  way  the  ore  becomes  uniformly  heated.  To 
obtain  a  satisfactory  result,  the  furnace  must  re- 


ROASTING    OF    OEES.  97 

volve  very  slowly.  It  makes  one  revolution  in  two 
to  five  minutes.  The  ore  is  carried  up  by  the  re- 
volving furnace  to  a  certain  height,  whence  it  falls 
through  the  flame.  The  draught  through  the  cylin- 
der carries  a  part  of  the  finest  ore  out  through  the 
flue  ;  it  is  therefore  necessary  to  build  dust-cham- 
bers between  the  flue  and  the  chimney.  On  the 
long  side  of  the  cylinder  is  an  opening,  closed  by 
a  lined  iron  door,  through  which  the  ore  is  dis- 
charged when  finished.  Through  this  same  door 
the  ore  is  charged.  This  kind  of  furnace  has  been 
in  use  at  the  La  Dura  works  for  several  years, 
where  it  has  given  satisfaction.  It  requires  less 
fuel  and  less  labor  than  the  single  reverberatory, 
and  only  of  late  has  it  been  replaced  by  a  long  fur- 
nace. Some  are  still  in  use  in  Colorado  Territory. 

57.  O'Hara's  Mechanical  Chain  Furnace.  Of  all 
furnaces,  the  object  of  which  is  a  continual  dis- 
charge of  roasted  ore,  taken  directly  from  the 
stamp^  without  the  intervention  of  manual  labor, 
O'Hara's  was  the  only  one  crowned  with  practical 
success.  Stetefeldt's  furnace  of  late  is  arranged  in 
the  same  way — that  is,  concerning  the  direct  feed- 
ing from  the  stamps  by  machinery.  O'Hara's  fur- 
nace is  in  use  on  Carson  River,  Nevada.  Three  of 
these  were  in  operation  at  Flint,  Idaho  Territory. 
At  present,  however,  they  have  stopped  for  want 
of  ore.  The  construction  of  O'Hara's  furnace  is 
shown  by  an  outline  drawing,  as  represented  in 
5 


98 


ROASTING    OF    ORES. 


Fig.  8.  The  hearth,  A,  is  104  feet  long  and  nearly 
five  feet  wide.  Eighty  feet  of  this  hearth  are 
crossed  by  an  arch,  B,  twelve  inches 
high,  and  connected  with  three  fire- 
places, two,  c  and  d,  on  one  side,  and 
one  between  c  and  d  on  the  other. 
a  is  the  feeding  hearth,  provided  with 
ore  continuously  from  the  batteries. 
The  motion  of  the  ore  is  effected 
by  an  endless  chain,  g,  passing  over 
two  chain  wheels,  one  at  each  end. 
To  this  chain  two  oblong  flat  rings, 
h,  are  attached,  each  provided  with 
eight  shovels  or  plows  so  arranged 
that  while  one  of  the  rings  shoves  the 
ore  toward  the  center  line,  the  other 
pushes  it  back  again  toward  the  sides 
every  three  or  four  minutes  (or  in 
shorter  intervals  if  more  ore  is 
charged).  The  ore  not  only  changes 
its  place  to  the  right  and  left,  but  it 
also  moves  forward  by  degrees,  so 
that  in  the  course  of  six  hours  from 
the  beginning,  it  commences  to  be 
discharged  at  f,  passing  eighteen 
"^  feet  over  the  cooling  hearth,  e.  On 
both  ends  of  the  furnace  are  iron 
doors  hung  on  hinges,  which  are 
opened  by  the  rings.  After  several 
months  of  operation  the  hearth  or 
bottom  appeared  in  good  condition. 


co 


ROASTING    OF    ORES.  99 

The  five  batteries,  five  stamps  each,  have  on 
both  long  sides  endless  screws,  by  which  the 
crushed  ore  is  forwarded,  in  proportion  as  it  is  dis- 
charged, to  an  elevating  apparatus.  Being  lifted 
about  fifteen  feet,  it  is  conveyed  again  by  endless 
screws  along  the  feeding  hearths  of  all  three  fur- 
naces, a',  and  regularly  divided  and  discharged  on 
the  feeding  hearth,  a.  The  ore,-  mixed  with  100 
pounds  of  salt  to  each  ton,  is  spread  on  iron  plates 
before  the  batteries,  (heated  by  the  hot  air  from  the 
furnaces,  conveyed  through  the  flue  and  under  the 
plates.)  When  charged  into  the  battery  the  ore  is 
not  further  handled  till  it  comes  out  of  the  furnaces 
perfectly  roasted  (§  32). 

There  is  only  one  obstacle  connected  with  this 
and  other  mechanical  furnaces.  The  shoes  or 
shovels,  touching  the  sides  of  the  furnace,  wear  off 
by  degrees,  leaving  a  space  which  is  taken  up  by 
the  ore.  This  part  of  the  ore  along  the  wall  hard-, 
ens  and  increases  in  amount  in  the  furnace  till  new 
shoes  are  put  in.  By  these  the  crust  of  one-half 
to  three-quarters  of  an  inch  thick  is  broken  off  and 
carried  out.  From  the  Rising  Star  ore  these  crusts 
contain  nearly  just  as  much  chloride  of  silver  as  the 
well  roasted  ore  ;  they  are,  nevertheless,  disagree- 
able, but  some  means  might  be  devised  by  which 
this  inconvenience  could  be  avoided. 

58.  Stetefeldt's  Coasting  Furnace.  This  furnace, 
now  being  built  at  Austin,  Nevada,  is  represented 
in  Fig.  9,  showing  a  vertical  cross  section.  The 


100 


ROASTING    OF    ORES. 


furnace  at  Reno,  Nevada,  has  a  dust-chamber  in 
place  of  the  flue,   b,   of   Fig.   9,  the  omission  of 


which  simplifies  the  construction  without  injury  to 
the  good  results  of  roasting.  The  furnace  has  three 
important  departments.  1st.  The  roasting  shaft, 


ROASTING    OF    OEES.  101 

a,  twenty-five  feet  high  and  five  feet  wide  at  the 
bottom,  narrowing  somewhat  toward  the  top,  to 
prevent  the  adherence  of  dust  to  the  wall.  It  is  a 
simple  shaft  of  common  bricks,  built  as  smooth  as 
possible.  On  the  top  of  the  shaft,  at  a',  is  placed 
an  iron  feeder,  through  which  a  permanent  and 
uniform  feeding  of  the  pulverized  ore,  already 
mixed  with  salt,  is  effected.  The  ore  falls  on  the 
bottom,  e,  and  when  half  a  ton  or  a  ton  is  accumu- 
lated, it  is  drawn  out  through  the  door,  f.  2d. 
The  fire-places.  There  are  three  gas  generators, 
constructed  similarly  to  that  of  the  copper-refining 
furnace  at  Mansfield,  Prussia.  The  cover  is  taken 
off  and  the  charcoal  introduced.  The  cover  is 
placed  again  on  its  frame,  which  contains  sand  in 
a  groove  in  order  to  shut  off  the  draft  entirely. 
The  slide  door  near  g  is  drawn  out,  and  the  char- 
coal falls  on  the  grate,  h,  through  which  as  much 
air  is  admitted  as  is  necessary.  There  are  also  two 
canals  on  each  side  of  the  grate,  oi\e  of  which  is 
shown  by  dotted  lines,  i,  both  communicating  at 
k.  Through  these  canals  is  regulated  the  admis- 
sion of  the  air  for  oxidizing  or  burning  the  car- 
bonic acid,  created  above  the  grate,  h.  In  the  flue, 
d,  air  and  gas  meet  together,  and  the  burning  pro- 
duct heats  the  furnace.  Two  of  these  generators 
heat  the  shaft,  a ;  the  mouth  of  one  is  shown  in 
the  drawing  by  c,  the  other  is  on  the  opposite  side, 
and  therefore  not  visible  in  the  plan.  The  two 
generators  are  constructed  exactly  like  g,  with  the 
exception  that  the  flue,  d,  is  not  inclined,  but  hori- 


102  BOASTING    OF    ORES. 

zontal.  The  flue,  d,  as  well  as  the  generators  above 
the  grates,  are  lined  with  fire-bricks.  3d.  The 
dust-chambers.  With  the  draft,  the  gases  from 
the  shaft,  with  a  part  of  the  fine  ore  dust,  pass 
through  the  vertical  flue,  b,  then  through  the  hori- 
zontal one,  m,  into  a  series  of  chambers,  ?i,  of  dif- 
ferent sizes.  The  first  four  chambers,  n,  are 
smaller  than  the  four  following,  which  are  not  rep- 
resented in  the  diagram  ;  from  the  last  chamber 
the  gases  draw  into  the  chimney.  The  dust  can  be 
removed  from  the  bottom  of  the  chambers  through 
the  doors,  o,  o.  Almost  all  the  dust  is  regained, 
and  not  in  a  raw  condition,  as  from  dust-chambers 
of  reverberatory  furnaces,  requiring  re-roasting, 
but  perfectly  chloridized,  which  is  principally  due 
to  the  auxiliary  generator,  gy  and  the  longer  con- 
tact with  the  chlorine  gases. 

Chimneys  and  Flues. 

59.  The  draft  in  a  furnace  depends  on  the 
height  of  the  chimney.  The  flue  or  canal  between- 
the  hearth  and  chimney  has  a  great  influence  on 
the  draft,  as  a  great  deal  of  heat  is  taken  up  by  the 
walls,  and  the  draft  in  the  chimney  depends  on  the 
temperature  therein  up  to  a  certain  degree.  It 
follows  that  the  longer  the  flues  are,  the  higher  the 
chimney  should  be.  Flues  underground,  once 
heated,  absorb  less  heat  than  if  exposed  to  the  air. 

Single  roasting  furnaces,  each  having  its  own 
chimney,  dispense  entirely  with  long  flues.  It  is 


BOASTING    OF    ORES.  103 

therefore  sufficient  to  build  the  chimney  twenty  to 
twenty-five  feet  above  the  level  of  the  hearth,  and 
fifteen  to  eighteen  inches  square  in  the  clear. 

Underground  flues  are  suitable  where  many 
roasting  furnaces  are  connected  with  the  chimney. 
They  are  often  built  directly  under  the  furnaces, 
two  feet  wide  by  three  to  four  feet  high.  In  this 
position  the  connections  between,  the  main  flue  and 
those  of  each  furnace  are  the  shortest.  Although 
this  mode  is  preferable  to  flues  on  the  side  of  the 
furnaces,  it  is  not  practicable  where  deep  ash-pits 
are  in  use.  Deep  ash-pits  are  "favorable,  partly  for 
the  reason  that  it  is  not  necessary  to  carry  out  the 
ashes  every  day  or  two,  but  principally  on  account 
of  the  grates,  which  last  longer,  being  better 
cooled  by  the  air. 

The  flues  are  sometimes  needlessly  carried  too 
far  out,  especially  when  it  is  intended  to  reach 
ascending  ground  on  which  the  flue  continues,  re- 
placing the  chimney  entirely  or  in  part.  The  ascent 
must  be  steep,  otherwise  if  the  length  is  in  no  pro- 
portion to  the  perpendicular  height  gained  by  it, 
taking  also  the  distance  from  the  furnaces  to  the 
ascending  ground  in  consideration,  the  absorption 
of  heat  might  neutralize  the  advantage  of  the 
ascending  flue.  On  the  other  hand,  the  tempera- 
ture in  the  chimney  should  not  exceed  300°  C.  = 
572°  F. 

A  chimney  fifty  to  seventy  feet  high,  and  from 
three  to  four  feet  square  inside,  is  generally  con- 
sidered sufficient  for  a  number  of  roasting  fur- 


104  EXTRACTION    OF    SILVER. 

naces.  It  is  built  of  common  bricks,  sometimes  of 
stone.  In  the  latter  case  the  stone  must  be  exam- 
ined to  ascertain  whether  it  will  stand  the  heat 
without  bursting.  The  stone  work  is  often  lined 
inside  with  a  layer  of  bricks. 

According  to  experience,  the  most  advantageous 
proportion  between  the  area  of  the  grate-openings 
and  the  section  of  a  chimney,  is  between  one  to 
one  and  two  to  one.  The  round  section  is  the 
most  proper,  as  there  is  the  least  friction  with  it. 
The  section  ought  to  be  the  same  at  all  heights. 


III.    EXTRACTION  OF  SILVER 

BY   WAY   OF   LIXIVIATION. 

Solving  Process. 

60.  Under  ' '  Solving  Process  "  is  to  be  under- 
stood here,  simply  roasting  with  salt,  and  extracting 
the  silver  with  hyposulphite  of  lime  or  of  soda, 
without  reference  to  particulars  of  roasting  in 
the  Patera  or  Kiss  processes. 

The  solving  process  comprehends,  generally  con- 
sidered, different  modes  of  extraction,  all  of  which 


EXTRACTION    OF    SILVER.  105 

are  based  on  the  property  of  the  solubility  of  the 
chloride  and  sulphate  of  silver.  The  extraction  of 
the  chloride  of  silver  by  alkaline  hyposulphites  was 
proposed  by  Percy.  Patera  was  the  first  who  made 
use  of  the  hyposulphite  of  soda  for  extraction  of 
silver  in  a  practical  way;  his  success,  however,  de- 
pends principally  on  his  modified  and  complicated 
roasting  (§  33).  By  lixiviation  the  silver  is  ex- 
tracted in  the  Patera,  Kiss,  Roszner-Patera,  Zier- 
vogel,  Augustin,  and  Kustel  &  Hofman  processes. 
The  extraction  of  silver  by  the  solving  process  is 
simple.  The  ore  is  first  roasted  with  salt  in  the 
usual  way,  whereby  the  formation  of  base  metal 
chlorides  cannot  be  avoided  entirely.  After  roast- 
ing, the  ore  is  first  subjected  to  leaching  with 
water,  in  order  to  extract  the  base  metal  chlorides, 
and  then  with  hyposulphite  of  lime,  to  extract  the 
silver. 

The  Extraction  of  Silver. 

61.  After  a  chloridizing  roasting  the  ore  should 
be  examined  to  ascertain  the  amount  of  chloride  of 
silver  contained  in  it,  according  to  §  21.  In  case 
the  extraction  should  not  be  satisfactoiy,  it  is  then 
easier  to  find  what  the  cause  is.  The  ore  is  then 
prepared  for  leaching. 

A.     First  Leaching.     The  roasted  ore  contains 

chloride  of  silver,  which  does  not  dissolve  in  water, 

but  generally  there  are  also  base  chlorides  in  it,  as 

the  chlorides  of  copper,  zinc,  lead,  iron,  antimony, 

5* 


106 


EXTRACTION    OF    SILVER. 


etc.,  which  are  soluble.  It  is  the  purpose  of  the 
first  leaching  to  extract  these  base  metals  by  means 
of  hot  water.  For  this  purpose  the  ore  is  intro- 
duced into  a  tub  or  square  box  of  pine  wood,  the 
planks  being  one  and  one-half  to  two  inches  thick. 
Fig.  10. 


The  boxes  must  be  made  as  water-tight  as  pos- 
sible and  provided  with  a  filter  at  the  bottom.  The 
filter  is  prepared  in  two  ways,  either  as  represented 
in  Fig.  10  by  fixing  a  false  bottom,  a,  provided 

Fig.  11. 


with  numerous  holes,  one-half  inch  in  diameter, 
about  one  inch  above  the  bottom,  b,  or  as 
Fig.  11  shows,  without  a  false  bottom.  On  the 
bottom,  a,  is  thrown  clean  rock,  quartz  or  poor  ore 
of  about  the  size  of  a  hen's  egg,  three  or  four  inches 


EXTKACTION    OF    SILVER.  107 

high  ;  on  this  smaller  stuff,  and  finally  sand,  free 
from  mud.  In  Fig.  11,  rock  of  about  the  same 
size  is  thrown  directly  on  the  bottom,  c,  spread 
four  inches  high,  then  a  few  buckets  full  of  rock, 
not  smaller  than  hazel  nuts,  and  only  so  much  of 
it  taken  as  to  equalize  the  surface.  This  is  then 
covered  with  a  piece  of  canvas  and  is  ready  for  use. 
The  boxes,  according  to  their  size  and  the  weight 
of  the  ore,  may  contain  from  one  to  five  tons. 
Generally  the  ore  must  not  be  over  fourteen  or  six- 
teen inches  deep,  but  some  ore  allows  a  good 
leaching  with  twenty-five  inches. 

The  roasted  ore,  generally  without  sifting,  is 
charged  into  the  boxes,  and  the  surface  spread 
evenly,  leaving  about  six  inches  space  from  the  top 
for  the  reception  of  the  leaching  water.  The  hot- 
ter the  water  is,  the  sooner  it  dissolves  the  soluble 
salts  and  the  quicker  the  leaching  progresses.  It 
is  conveyed  through  the  pipe,  d,  and  falls  on  a 
piece  of  canvas,  whence  it  spreads  equally  and 
gently  over  the  ore.  The  water  soon  reaches  the 
bottom  and  begins  to  flow  out  through  the  pipe,  e, 
into  the  trough,  f. 

In  the  beginning,  the  leaching  water  at  e  is 
highly  charged  with  base  metal  salts,  and  shows  a 
green  color  if  there  is  much  copper  in  the  ore. 
The  water  is  kept  running  in  a  continual  stream 
till  it  reaches  nearly  to  the  rim  of  the  box,  when 
the  influx  and  the  efflux  are  equalized.  After  one 
or  two  hours  a  glass  full  of  the  liquid,  at  the  pipe, 
e,  is  taken,  and  a  few  drops  of  sulphide  of  calcium 


108  EXTRACTION    OF    SILVER. 

(or  of  sodium)  added.  If  a  precipitate  falls,  of  a 
dark  or  light  color,  the  leaching  must  continue  ; 
but  it  is  not  necessary  to  continue  until  no  precipi- 
tate at  all  is  perceived,  as  it  requires  some  time — 
perhaps  an  hour — before  all  the  water  runs  out 
after  the  pipe,  d,  is  closed.  The  water  which 
comes  out  last  must  be  free  from  salts.  This  first 
leaching  takes  from  two  to  four  hours,  sometimes 
longer. 

B.  Second  Leaching.  As  soon  a*s  the  ore  is 
freed  from  the  base  chlorides  soluble  in  water,  a 
solution  of  hyposulphite  of  lime  (§  70)  is  led  in 
from  a  tub  or  tank,  on  the  ore,  in  order  to  dissolve 
the  chloride  of  silver.  This  leaching  is  conducted 
like  the  former.  It  depends  on  the  amount  of 
silver  how  long  this  work  continues — from  eight  to 
twenty  hours.  The  clear  cold  solution,  containing 
the  chloride  of  silver  in  the  form  of  a  double  salt, 
has  a  very  sweet  taste,  and  is  conveyed  through  a 
trough  or  india  rubber  hose  into  a  precipitating 
tub.  Very  rich  ore,  containing  12  to  15  per  cent, 
of  silver,  would  require  forty-eight  hours  leaching, 
and  even  then  it  would  be  necessary  to  subject  the 
ore  to  a  second  leaching  with  the  hyposulphite, 
with  an  intermediate  roasting  with  green  vitriol 
and  salt ;  for,  with  the  best  work,  if  95  per  cent, 
are  extracted,  the  tailings  would  still  appear  suffi- 
ciently rich  for  this,  containing  about  200  ounces 
of  silver  per  ton.  Ores  containing  $350  per  ton 
are  often  leached  out  perfectly  in  twelve  hours. 
The  end  of  the  lixiviation  is  ascertained  in  the 


EXTRACTION    OF    SILVER.  109 

same  way  as  in  leaching  with  water,  using  the  sul- 
phide of  calcium.  If  no  precipitate  is  obtained 
the  extraction  is  finished. 

The  color  of  the  precipitate  is  a  black-brown. 
The  presence  of  base  metals  changes  the  color 
somewhat.  Iron  makes  it  black ;  copper,  red- 
brown  ;  lead  and  antimony,  light  red-brown,  etc. 
The  silver  is  first  dissolved,  especially  if  a  diluted 
solution  of  hyposulphite  of  lime  is  used  ;  and  for 
this  reason  the  first  precipitate  is  the  richest  in 
silver.  Ore  containing  a  great  deal  of  lead — 
especially  if  the  roasting  was  so  conducted  that  a 
large  part  of  it  remained  as  sulphate  of  lead,  which 
is  not  soluble  in  the  leaching  water — will  give  in 
the  beginning  of  the  leaching  with  the  solvent  a 
precipitate  of  silver  with  some  lead  ;  afterwards, 
however,  the  silver  diminishes,  so  that  the  precipi- 
tate of  lead  finally  appears  free  of  silver.  Besides 
the  sulphate  of  lead,  sub-chlorides  and  oxy-chlo- 
rides  are  formed  during  the  roasting  which  are  not 
soluble  in  water,  but  are  dissolved  by  the  hyposul- 
phite of  lime  ;  for  this  reason  always  some  base 
metals  will  be  found  in  the  precipitate. 

In  case  rebellious  ores  are  treated,  and  hot  water 
is  used  for  the  extraction  of  base  chlorides,  a  bet- 
ter silver  is  obtained  if  the  ore  is  cooled  down  by 
cold  Water  before  the  cold  and  diluted  solvent  is 
applied.  Purer  ores  may  be  treated  with  a  warm 
solution  of  the  solvent. 

When  examining  the  tailings  as  to  the  amount 
of  silver  left  therein,  it  must  be  remembered  that, 


110  EXTRACTION    OF    SILVER. 

after  leaching  out  a  quantity  of  metals  by  water 
and  the  solvent,  the  ore  lost  a  considerable  part  of 
its  original  weight,  and  that  consequently  one-half 
ounce  of  such  tailings  taken  into  assay  will  always 
give  a  larger  silver  button  than  there  ought  to  be. 
A  true  assay  of  leached  tailings  is  made  if  half  an 
ounce  of  the  same  ore  is  leached  on  a  filter  with 
hot  water  and  hyposulphite  of  lime,  in  the  same 
way  as  the  ore  on  a  large  scale,  washed  with  water, 
dried  and  weighed.  The  weight  found  after  leach- 
ing must  be  taken  for  half  an  ounce  in  assaying 
the  tailings. 

The  residue,  or  tailings  in  the  leaching  box,  must 
be  removed  now  as  valueless.  The  sides  of  the 
leaching  boxes  are  from  eighteen  to  twenty-four 
inches  above  the  bottom,  and  being  from  six  to 
eight  feet  square  in  the  clear,  the  removing  of  the 
tailings  by  means  of  chloride  is  easily  effected. 
The  men  must  be  careful  not  to  dig  too  deep, 
otherwise  the  filter  will  be  injured.  It  is  quite 
proper  to  fix  wooden  staves,  as  long  as  the  box  re- 
quires, on  top  of  the  filter.  These  staves  are  one 
inch  wide  and  one-half  of  an  inch  thick,  and  are 
placed  from  four  to  five  inches  apart,  so  as  to  pro- 
tect the  canvas  or  filter  against  the  shovel.  In 
Fig.  11  the  staves  are  laid  upon  the  canvas. 

The  leaching  boxes  or  tubs  may  be  arranged  so 
that,  being  tipped  over,  the  whole  charge  falls  out 
at  once.  In  this  case  the  filter  must  be  made  in  a 
different  way  from  that  described  above.  Eocks 
are  not  serviceable  here.  On  the  false  bottom,  n, 


EXTRACTION    OF    SILVER. 


Ill 


of  Fig.  10,  a  layer  of  thin,  leafless  switches  is 
placed,  and  on  this  another  one  crosswise,  then 
covered  with  a  piece  of  canvas,  and  secured  with 
some  staves  to  prevent  the  falling  out  of  the  whole 
filter  when  turned  over. 


Precipitation  of  the  Silver. 

62.     The  liquid  of  the  second  leaching  is  con- 
veyed through  a  trough  or  india  rubber  hose  to  the 

Jfiff.  12. 


precipitating  tanks,  of  which  three  or  four  are  em- 
ployed. If  tubs  are  used,  which  for  this  purpose 
are  the  best,  they  are  from  three  to  four  feet  in 
diameter,  and  four  feet  high.  The  tanks  or 


OF  THB 

TJNIVERSIT 


:  EXTRACTION    OF    SILVER. 

boxes  have  a  rectangular  shape  of  about  the  same 
capacity,  the  bottom  being  inclined  toward  the 
middle,  as  shown  by  Fig.  12.  The  hyposulphite 
of  lime,  as  it  comes  from  the  leaching  tanks,  is 
conducted  into  these  until  they  are  more  than  two- 
thirds  full.  .  The  trough  or  hose  is  then  changed 
to  discharge  the  liquid  into  the  next  precipitating 
tub,  while  the  precipitation  of  the  first  commences. 
The  liquid  used  for  precipitating  the  silver  is 
sulphide  of  calcium  (§  69).  It  is  poured  in  until 
all  the  silver  is  supposed  to  be  precipitated,  and  at 
the  same  time  the  solution  is  stirred  vigorously. 
Treating  always  the  same  kind  of  ore,  the  required 
quantity  of  the  precipitating  agent  is  soon  learned. 
The  black  precipitate  sinks  to  the  bottom,  and  the 
workman  now  dips  a  little  of  the  clear  liquid  'out 
in  a  glass  tube  or  tumbler,  and  adds  a  few  drops  of 
the  sulphide  of  lime.  If  a  dark  precipitate  or  a 
dark  color  is  produced,  it  shows  that  there  is  still 
silver  in  the  liquid,  and  more  of  the  agent  must  be 
added  ;  but  if  on  the  contrary  no  precipitate  is  ob- 
served, there  is  either  enough  or  too  much  of  the 
sulphide.  To  prove  this,  some  of  the  silver-hold- 
ing liquid  is  added  to  a  test,  taken  from  the  tank 
under  treatment.  If  in  this  case  a  precipitate  is 
formed,  silver-holding  liquid  must  be  carefully 
added  to  the  tank  until  no  reaction  is  produced. 
This  work,  delicate  as  it  seems,  is  easily  learned  by 
the  workmen.  If  a  little  silver  should  be  left  in 
the  liquid,  it  is  not  injurious,  neither  is  the  silver 
to  be  considered  as  lost,  because  the  same  liquid  is 


EXTRACTION    OF    SILVER.      '  113 

used  over  again  ;  but  a  small  excess  of  the  sulphide 
of  calcium  would  cause  a  loss  in  silver,  as  it  pre- 
cipitates sulphide  of  silver  in  the  leaching  tank  in 
the  mass  of  ore,  which  is  not  dissolved  again.  The 
precipitation  is  performed  in  a  short  time,  requir- 
ing about  fifteen  minutes  for  each  tank.  The  stir- 
ring must  be  executed  with  vigor.  Wooden  grates 
fixed  to  a  vertical  stem  will  answer  the  purpose. 

The  clear  solution  above  the  settled  precipitate 
is  pumped  or  elevated  to  the  reservoir,  whence  it 
was  conveyed  on  the  ore.  It  is  now  ready  to  be 
used  again.  The  sulphide  of  calcium  having  per- 
formed its  duty  in  precipitating  the  silver,  is  turned 
into  hyposulphite  of  lime,  thus  replacing  all  of  the 
solvent. 

To  prevent  small  floating  particles  of  silver  from 
being  elevated  with  the  liquid,  it  is  well  to  allow 
sufficient  time  for  the  precipitated  silver  to  settle. 
For  this  reason  it  is  better  to  have  more  precipi- 
tating tanks  or  tubs.  It  is  not  necessary  to  re- 
move the  silver  after  each  precipitation.  The  clear 
liquid  can  be  drawn  off,  by  means  of  a  syphon, 
from  all  the  precipitating  tubs  into  a  general  re- 
ceiver, whence  it  may  be  .pumped  up.  After  the 
solvent  has  been  removed,  the  precipitated  silver 
can  be  drawn  off  through  the  pipe,  d,  Fig.  12,  di- 
rectly into  canvas  bags. 

Treatment  of  the  Precipitated  Silver. 

63.  The  black  precipitate  of  sulphide  of  silver 
is  conveyed  directly  into  filters  made  of  canvas, 


114  EXTRACTION    OF    SILVER. 

either  in  the  shape  of  pointed  bags,  like  those  used 
for  amalgam,  or  in  the  shap'e  of  common  bags.  As 
soon  as  all  the  liquid  runs  out,  pure  water  (if  pos- 
sible, warm)  is  poured  on  the  silver,  and  this  re- 
peated several  times  till  no  taste  is  observed  in  the 
filtering  water.  The  precipitate,  while  still  in  the 
bags,  is  placed  beneatn  a  screw  press  and  the  fluid 
pressed  out  as  completely  as  possible.  The  black 
silver  cakes  are  then  taken  out  and  dried  in  a  warm 
room  or  in  a  drying  oven.  For  the  purpose  of 
burning  off  the  sulphur,  the  dried  sulphide  is  in- 
troduced into  a  muffle  or  other  calcining  furnace, 
and  heated  till  the  sulphur  commences  to  burn 
with  its  known  blue  flame.  When  this  disappears 
the  heating  must  continue  at  a  dark  red  heat  for 
one  or  two  hours.  By  this  operation  the  cakes  are 
reduced  almost  entirely  to  metallic  silver,  generally 
covered  with  threads  of  silver  ;  sometimes  an  in- 
tense green  color  is  assumed  by  pieces  remaining 
in  the  furnaces  over  night. 

The  burned  cakes  are  now  prepared  for  smelting 
in  crucibles.  They  are  placed  in  black  lead  cru- 
cibles, according  to  the  size,  up  to  three  hundred 
pounds,  and  fused.  All  the  sulphur  was  .  not 
driven  out  by  the  preceding  operation.  The  re- 
maining part  must  be  removed  by  placing  metallic 
iron  (§  5;  d)  in  the  fused  metal  ;  thereby  iron  matt 
is  formed,  which  rises  to  the  surface  and  is 
skimmed  off.  The  surface  of  the  silver  is  then 
cleaned  by  adding  some  bone  ash  and  borax,  or 
borax  alone,  which  is  also  skimmed  off  and  the 


EXTRACTION    OF    SILVER.  115 

silver  dipped  out  or  poured  out  into  moulds.  Ac- 
cording to  the  careful  treatment  in  the  roasting 
process,  and  the  nature  of  the  ore,  the  silver  will 
be  from  800  to  950  fine. 

Mr.  O.  Hofmann,  in  need  of  sulphur  for  the 
production  of  sulphide  of  calcium,  used  to  calcine 
the  dried  sulphide  of  silver  in  iron  retorts.  In  this 
way  he  obtained  a  large  proportion  of  sulphur  as  a 
fine  sublimate.  This  could  be  done  also  in  a  proper 
muffle  furnace,  so  arranged  that  after  all  obtaina- 
ble sulphur  had  sublimated  in  a  receiver  this  could 
be  removed  and  the  calcination  continued  under 
access  of  air. 

Precipitation  of  Copper  contained  in  the 
Ore  and  of  a  small  amount  of  Silver 
leached  out  with  the  Copper. 

64.  Having  refractory  ore  under  treatment,  it 
is  generally  the  case  that  copper  is  also  found  in  it. 
While  roasting,  the  presence  of  copper  is  favorable 
for  the  chlorination  of  the  silver,  but  copper  ores 
require  some  more  salt,  especially  if  it  is  intended 
to  save  the  copper  also.  The  more  chloride  of 
copper  formed,  the  more  will  be  found  in  the  so- 
lution while  leaching  it  with  hot  water.  In  order 
to  convert  all  the  copper  into  a  chloride,  it  would 
take  at  least  one  and  a  half  pounds  of  salt  to  each 
pound  of  copper ;  and  considering  other  base 
metals,  lime,  etc.,  all  of  which  absorb  chlorine, 
while  a  considerable  part  escapes  useless,  the  above 


116  EXTRACTION    OF    SILVER. 

quantity  has  to  be  doubled.  For  this  reason  no 
special  attention  can  be  paid  to  the  copper  ;  only 
that  part  of  it  can  be  extracted  which  is  converted 
into  a  chloride  during  roasting  under  the  usual 
circumstances.  The  chloride  of  copper  transfers  a 
part  of  its  chlorine  to  the  silver  and  other  metals 
(§  23),  and  is  reduced  thereby  to  a  sub-chloride  ; 
if  there  is  sufficient  salt  in  the  furnace  it  is  raised 
again  to  a  chloride.  This  sub-chloride  (Cu2  Cl)  is 
not  soluble  in  water ;  it  remains  in  the  ore  during 
leaching. 

65.  The  different  chlorides,  being  removed  in 
the  first  leaching  (§  61),  are  principally  those  of 
copper,  iron,  lead,  antimony  and  zinc,  besides  some 
undecomposed  salt.  The  first  quantity  of  hot 
water  introduced  into  the  leaching  box  is,  of 
course,  most  saturated  with  the  named  salts,  and 
they  have  the  property  of  dissolving,  also,  some 
chloride  of  silver.  The  dissolved  silver  precipi- 
tates again  as  soon  as  it  becomes  diluted  with  more 
water.  There  is,  therefore,  no  diniculty  in  regain- 
ing the  silver  which  is  thus  leached  out.  The 
amount  of  silver  carried  out  by  the  leaching  water 
varies  from  0.5  to  three  per  cent.  Not  only  the 
chloride  of  silver,  but  also  those  of  lead  and  anti- 
mony, are  precipitated  by  dilution  with  water. 
There  are  two  ways  of  regaining  this  silver. 

Mr.  O.  Hofmann  adopted  an  ingenious  plan  for 
this  purpose,  by  conveying  the  hot  water,  under  a 
slight  pressure  from  below,  through  the  pipe,  e, 


EXTK ACTION    OF    SILVEE.  117 

(§  61,  Fig.  10),  by  attaching  to  it  a  rubber  hose. 
The  water  rises  through  the  ore  from  e  up  to  d,  and 
as  soon  as  it  reaches  within  two  inches  of  the  brim 
of  the  box,  the  hose  is  removed  from  e,  and  the 
water  admitted  through  d.  The  concentrated  so- 
lution, containing  dissolved  silver,  is  now  above 
the  ore,  and  being  diluted  with  water  from  d,  lets 
the  chloride  of  silver  fall  as  a  precipitate  on  and 
throughout  the  ore. 

The  other  plan  is  the  precipitation  of  the  silver, 
together  with  the  chlorides  of  lead  and  antimony, 
outside  of  the  leaching  box.  This  mode  is  prefer- 
able to  the  former  when  a  great  deal  of  lead  and 
antimony  is  in  the  ore  ;  for  if  precipitated  in  the 
box,  a  great  part  of  it  will  be  dissolved  by  the  hy- 
posulphite of  lime  and  then  precipitated  as  sul- 
phides with  the  silver,  making  this  impure  and  con- 
suming much  of  the  precipitating  agent.  As  soon 
as  the  chlorides  flow  into  the  trough,  f,  below  e, 
into  which  several  leaching  boxes  discharge  their 
fluids  in  different  degrees  of  dilution,  the  gradual 
precipitation  commences.  The  precipitate  is  white 
and  adheres  to  the  trough  through  the  whole 
length  of  it.  These  chlorides  are  the  richest,  and 
contained,  at  Flint,  Idaho,  9  per  cent,  of  silver;  the 
balance  was  principally  lead  and  antimony.  The 
precipitate  deposits  on  all  bodies  offering  a  surface. 
For  this  purpose  a  box  must  be  constructed,  as 
represented  in  Fig.  13,  which  shows  the  top  view 
or  ground  plan.  The  sides,  a,  of  a  wooden  box, 
six  feet  by  six,  or  ten  by  six,  are  six  inches  high, 


118 


EXTRACTION    OF    SILVER. 


the  front,  b',  and  partitions,  b,  four  inches  high, 
leaving  a  space  of  six  inches  between  them.  These 
spaces  are  filled  with  shavings  representing  an  im- 
mense amount  of  surface  for  the  chlorides  to  de- 
Fig.  13. 


I 
3k 

,i 

^ 


j 


posit  on.  The  fluid  entering  the  trough,  c,  contains 
now  a  purified  copper  solution.  Chloride  of  iron 
is  also  with  the  copper  in  solution,  but  does  not 
prevent  the  copper  from  precipitating. 

66.  The  white  precipitate,  when  accumulated, 
is  taken  out,  placed  in  filtering  bags,  with  or  with- 
out the  shavings,  and  washed  with  clear  cold  water, 
in  order  to  get  rid  of  the  copper  solution.  The 
silver  can  be  extracted  in  two  ways  :  The  simplest 
mode  is  the  application  of  hyposulphite  of  lime. 


EXTRACTION    OF    SILVER.  119 

The  sediment  is  taken  out  from  the  filtering  bags 
and  charged,  while  wet,  into  a  filtering  box  of  a 
proper  size,  arranged  like  Fig.  11,  §  61.  The 
hyposulphite  of  lime,  in  a  cold  condition,  is  poured 
over  it  and  managed  as  with  the  ore  with  the 
second  leaching,  §  61.  The  silver-holding  fluid 
may  be  conveyed  into  the  precipitating  box,  Fig. 
12,  §  62,  and  treated  with  the  solution  from  the 
ore.  The  liquid  from  the  bags  is  examined  from 
time  to  time  with  sulphide  of  calcium.  In  the  be- 
ginning the  precipitate  appears  dark,  being  mostly 
silver ;  but  when  it  is  perceived  that  the  precipi- 
tate assumes  a  light  yellow  color,  too  much  of  lead, 
zinc  and  antimony  is  being  carried  out,  and  the 
leaching  must  be  stopped.  The  residue  in  the  fil- 
ter box  contains  still  some  silver. 

The  other  mode  of  extraction  is  more  perfect, 
but  also  more  expensive  and  more  troublesome. 
After  the  copper  has  been  washed  off,  the  contents 
of  the  bags  are  taken  out  and  dried.  It  is  then  in- 
troduced into  large  crucibles  and  smelted  with  an 
addition  of  soda-ash.  The  reduced  metal,  if  some 
lead  occurs  in  the  ore,  must  be  separated  by  means 
of  cupellation,  resulting  in  clean  silver  and  litharge. 

67.  The  chloride  of  copper  running  from  the 
box,  Fig.  13,  is  led  into  a  reservoir  in  which  old 
iron  is  suspended.  The  copper  precipitates  in  a 
metallic  state  on  the  iron,  and  about  eighty-eight 
parts  of  iron  go  into  the  solution  in  place  of  one 
hundred  parts  of  copper ;  consequently,  as  each 


120  EXTRACTION    OF    SILVER. 

one  hundred  pounds  of  pure  copper  require  eighty- 
eight  pounds  of  iron,  the  calculation  as  to  the 
necessary  amount  of  iron  could  be  made  easily  if  it 
were  not  for  some  other  chlorides  which  may  still 
be  in  solution,  and  which  also  require  iron  for  pre- 
cipitation. Wrought  iron  is  preferable  to  cast 
iron,  and  gray  cast  iron  is  better  than  white;  but 
all  these  sorts  precipitate  the  copper,  and  it  de- 
pends to  a  great  extent  on  the  price  as  to  what 
kind  of  iron  is  chosen. 

The  most  effective  precipitant  is  the  iron-sponge 
or  finely  divided  iron,  obtained  by  heating  pulver- 
ized iron  ore  or  roasted  iron  pyrites  with  charcoal 
powder  in  a  proper  reverberatory  furnace,  or  in 
iron  pipes  or  cylinders  without  admitting  air.  By 
these  means  the  iron  oxide  is  reduced  to  metallic 
iron,  which  precipitates  the  copper  in  a  few 
minutes.  Using  old  iron,  the  precipitation  will  be 
effected  much  more  quickly  than  in  tanks  if  an  ar- 
rangement is  made  like  Fig  13,  putting  the  old 
iron  in  the  place  of  the  shavings.  This  would  be  a 
continuation  of  Fig.  13,  but  the  box  must  be  three 
or  four  times  as  long  before  it  reaches  the  tank  or 
reservoir. 

To  find  out  whether  there  is  yet  copper  in  the 
liquid,  the  best  test  is  to  take  some  drops  on  a 
piece  of  platinum,  and  to  place  a  small,  clean  piece 
of  zinc  on  it.  The  copper  immediately  appears  of 
a  bright  red  color.  But  for  practical  use,  a  clean 
piece  of  iron  dipped  into  the  liquid  will  also  show 
a  red  coating  if  there  is  enough  copper  in  it  to 


EXTRACTION    OF    SILVER.  121 

make  it  remunerative  to  continue  the  precipitation. 
The  water  contains  now  principally  chloride  of 
iron,  and  is  discharged.  If  by  some  cheap  means 
the  water  could  be  evaporated,  the  remaining  chlo- 
ride of  iron  could  be  used  in  roasting  ores  without 
salt. 

Where  old  iron  commands  a  high  price,  the  cop- 
per can  be  precipitated  with  a  brine  of  ashes  or  of 
lime  ;  but  in  this  case  the  iron  also  falls  with  the 
copper.  The  brine  for  this  reason  cannot  be  ad- 
vantageously adopted  where  a  great  deal  of  iron  is 
in  the  ore,  or  the  roasting  must  be  directed  so  as 
to  decompose  the  chloride  of  iron. 

Quality  of  Ores  fit  for  the  Solving  Process. 

68.  There  is  no  process  so  suitable  for  all  kinds 
of  ores  as  the  solving  process.  G-enerally  consid- 
ered, all  silver  ores  can  be  treated  by  the  solving 
process  which  are  subjected  to  the  pan  amalga- 
mation after  roasting  ;  but  in  many  instances — 
especially  with  the  rebellious  ores — a  better  result 
is  obtained  by  this  than  by  working  in  pans.  The 
great  advantage  of  this  process  is  cheapness. 
Boasting  of  course  is  indispensable  except  with 
chloride  ores  ;  but  neither  pans  and  the  required 
power,  nor  quicksilver,  are  used,  and  for  this  reason 
less  capital  is  necessary  to  put  up  reduction  works. 
All  the  cupreous  silver  ores  of  Cerro  Gordo,  Yellow 
Pine,  Montgomery,  and  of  the  other  new  silver 
districts,  can  be  treated  to  great  advantage  by  the 
solving  process. 
6 


122 


EXTRACTION    OF     SILVER. 


rr 

1 _\ 


Two  objections  have  to  be  considered.  First, 
there  is  more  water  required  than  for  pan  amalga- 
mation— at  least  this  is  the  case  with  rebellious 

ores ;  but  the  quantity 
of  water  depends  on  the 
quantity  of  base  metals 
in  the  ore,  and  also  on 
the  arrangement  of  the 
leaching  boxes.  One 
box,  containing  one  ton 
of  ore,  requiring  three 
hours  leaching,  may  con- 
sume 250  gallons  of 
water ;  three  boxes  of 
the  same  size  would 
take  three  times  as  much 
water  if  placed  on  the 
same  level ;  but  by  ar- 
ranging the  boxes  in  a 
less  favorable  position, 
one  above  the  other,  as 
shown  in  Fig.  14,  only 
one-half  of  the  quantity 
of  water  is  needed.  It 
takes  more  time  to  leach 
three  boxes  together  than 
a  single  one.  Leach- 
ing the  ore  with  hypo- 
sulphite of  lime,  the  supply  of  this  in  the  first  box 
must  be  stopped  if  no  more  silver  comes  out,  and  the 
solution  carried,  by  means  of  hose,  to  the  second, 


EXTRACTION    OF     SILVER.  123 

and  then  to  the  third  box,  if  it  should  be  neces- 
sary; so  that  while  the  second  box  is  yet  under 
leaching,  the  first  can  be  discharged  and  a  new 
charge  introduced.  All  three  boxes  should  receive 
clean  water  at  the  start.  This  arrangement  should 
be  adopted  only  when  rendered  necessary  by  the 
scarcity  of  water.  To  have  all  leaching  boxes  on 
a  level  is  preferable. 

The  other  objection  is  confined  to  a  certain  class 
of  ores  containing  clay  and  lime.  If  pulverized, 
so  much  fine  pulp  will  be  produced  that  the  leach- 
ing is  impossible.  It  is  not  advisable  to  crush  the 
ore  coarser  than  will  allow  of  its  passing  through  a 
sieve  of  forty  holes  to  the  inch,  in  some  cases, 
perhaps,  through  thirty-five  holes  ;  and  if  with 
such  crushing  a  fine  clay  pulp  is  produced,  the  ore 
is  unfit  for  all  leaching  processes,  unless  wet  crush- 
ing is  adopted,  in  order  to  separate  the  slime  from 
the  sand,  as  Mr.  O.  Hofmann  was  compelled  to 
arrange  in  Trinidad,  Sonora.  In  this  case,  a  sepa- 
rate drying  for  the  purpose  of  roasting  is  not 
necessary  if  long  furnaces  are  in  use.  It  is  not 
unlikely  that  for  similar  ore  and  pan  tailings  an 
agitating  filtering  box  could  be  constructed  which 
would  render  the  leaching  possible. 

Sulphide  of  Calcium. 

69.  Sulphide  of  calcium  for  the  precipitation 
of  silver  is  preferable  to  the  sulphide  of  sodium, 
principally  for  the  reason  that  its  manufacture  is 


124  EXTRACTION    OF     SILVER. 

cheaper  and  more  eas}73  but  also  on  account  of  the 
quality  of  the  precipitated  silver,  which  is  easier  to 
wash,  to  press  and  to  desulphurize.  The  sulphide 
of  calcium  is  easily  obtained  and  manufactured  on 
the  ground  where  the  mill  is  situated.  The  articles 
required  for  this  purpose  are  brimstone  (worth  about 
four  cents  per  pound)  and  burned  lime.  The  sul- 
phide is  formed  only  from  caustic  lime,  consequently 
more  is  obtained  from  fresh  burned  lime.  Of  this 
a  certain  quantity  is  charged  into  an  iron  kettle, 
water  added,  and  then  the  pulverized  sulphur.  The 
proportion  of  sulphur  and  lime  depends  on  the 
quality  of  the  latter.  The  purest  quality  of  lime 
from  Santa  Cruz,  Cal.,  for  instance,  takes  one 
pound  of  sulphur  to  1.33  of  lime.  Of  poorer  quali- 
ties of  lime  it  is  better  to  take  three  pounds  to  one 
of  sulphur  and  about  ten  parts  of  water.  It  is 
kept  boiling  for  two  or  three  hours,  stirred  with 
woo.den  shovels,  and  then  discharged  into  a  filter- 
ing box,  prepared  like  Fig.  10,  §  61.  The  clear, 
dark,  yellow-red  sulphide  of  calcium  comes  out 
from  below  the  filter,  and  can  be  kept  in  iron  ves- 
sels. The  liquid  ought  to  be  between  5°  and  6° 
Beaum4.  The  residue  is  washed  with  water, 
whereby  a  diluted  fluid  is  obtained,  which  is  used 
with  the  lime  of  the  next  charge.  Mr.  E.  Smyth, 
in  La  Dura,  Mexico,  treats  the  lime  and  sulphur 
with  steam.  This  has  the  advantage  of  dispensing 
with  the  stirring,  and  may  be  performed  also  in 
wooden  vessels.  The  steam  replaces  the  fire  and 
has  no  chemical  influence  on  the  quality  of  the  sul- 


EXTRACTION    OF     SILVER.  125 

pliide  ;  the  precipitating  capacity  of  the  latter  with 
reference  to  the  volume  depends  only  on  the  con- 
centration which  is  expressed  by  the  degrees  of 
Beaume's  hydrometer. 

One  pound  of  lime  (Santa  Cruz  quality)  gives 
sufficient  sulphide  of  calcium  to  precipitate  one 
and  a  half  pounds  of  silver. 

Hyposulphite  of  Lime. 

70.  The  hyposulphite  of  lime  as  a  solvent  of 
chloride  of  silver  has  a  great  advantage  over  a  hot 
solution  of  salt.  It  can  be  applied  diluted  and 
cold,  and  dissolves  a  great  deal  more  of  the  chlo- 
ride than  does  the  salt,  of  which  nearly  sixty-eight 
pounds  are  required  to  dissolve  one  pound  of  chlo- 
ride of  silver,  while  only  two  pounds  of  the  hypo- 
sulphite are  needed  to  dissolve  the  same  amount  of 
the  chloride. 

The  hyposulphite  of  lime  is  produced  by  convey- 
ing sulphurous  acid  into  sulphide  of  calcium  till 
it  appears  entirely  colorless.  It  is  also  formed  if 
from  a  concentrated  brine,  obtained  from  lixivi- 
ating roasted  ore  with  hot  water,  all  chlorides  are 
precipitated  by  sulphide  of  calcium.  After  the 
precipitated  sulphides  have  settled,  the  clear  fluid 
can  be  used  to  dissolve  the  chloride  of  silver.  The 
simplest  way,  however,  is  to  buy  hyposulphite  of 
soda,  and  to  commence  the  second  leaching  (§  61) 
therewith,  precipitating  with  sulphide  of  calcium. 


126  EXTRACTION    OF     SILVER. 


Patera  Process. 

71.  The  most  delicate   operation   in   Patera's 
-process  is  the  preparation  by  roasting,  as  described 
in  §  33.     The  chloride  of  silver  formed  during  the 
roasting  is  dissolved  by  a  cold  solution  of  hyposul- 
phite of  soda,  after  all  soluble  base  metals  have 
been  first  leached  out  with  hot  water  (§  61).    'Two 
parts  of  the  hyposulphite  of  soda  dissolve  one  part 
of  chloride  of  silver,  forming  a  soluble  double  salt. 
The  tubs  in  which  the  ore  is  lixiviated  with  the  hy- 
posulphite of  soda  are  small,  receiving  only  200 
pounds  of  roasted  ore.     The  extraction  of  silver  is 
performed  in  the  same  way  as  described  under  §  61 . 

Kiss   Process. 

72.  This  process  extracts  silver  and  gold  to- 
gether.    Boasting  the  ore,  as  explained  in  §  38, 
the  gold  is  transformed  into  such  a  state  as  to  ren- 
der it  insoluble  in  water.     After  roasting,  the  ore 
is  placed  in  filtering  tubs  and  washed  with  water 
to  remove  the  base  metals.     After  this  a  solution 
of  hyposulphite  of  lime  is  conveyed  on  the  ore,  by 
which  the  gold  and  silver  chlorides  are  dissolved 
and  carried  off  into  precipitating  tubs.     As  soon 
as  the  sulphide  of  calcium  is  introduced,  the  gold 
and  silver  are  precipitated  as  sulphides.     The  pre- 
cipitation of  both  metals  in  a  metallic  condition  is 
not  admissible,  for  the  reason  that  the  hyposul- 


EXTRACTION    OF    SILVER.  127 

phite  of  lime  is  decomposed  if  metallic  copper  is 
employed  for  precipitation.  The  results  of 'Kiss's 
methods,  practiced  in  Hungary,  were  not  altogether 
satisfactory. 

Patera  and  Rceszner  Processes. 

73.  The  object  of  these  processes  is,  like  that  of 
the  preceding,  the  extraction  of  silver  and  gold 
together.  The  ore  is  first  subjected  to  a  chloridiz- 
ing  roasting,  by  which  the  silver  is  converted  into 
a  chloride,  while  the  gold  remains  mostly  in  me- 
tallic condition.  The  leaching  liquid  is  prepared 
by  conveying  chlorine  gas  through  a  cold  concen- 
trated solution  of  salt  to  saturation.  This  chlo- 
ridized  solution  dissolves  silver,  gold  and  copper 
at  the  same  time.  The  roasted  ore  is  charged  into 
tubs  with  false  bottoms,  and  the  cold  solution  of 
salt  and  chlorine  introduced.  Silver  ores  treated 
after  this  method  in  Hungary  gave  98.94  per  cent, 
of  the  silver,  all  the  copper  and  nearly  all  the 
gold.  An  experiment  on  five  tons  of  ore  gave  a 
clear  profit  of  seventy-five  florins,  compared  with 
the  amalgamation. 

Roszner  roasts  the  ore  with  salt,  extracts  a  part 
of  the  silver  by  Augustin's  method  with  a  hot  so- 
lution of  salt,  and  treats  the  residue  alternately 
with  a  solution  of  salt  and  chlorine,  and  with  a  hot 
concentrated  salt  solution  for  the  extraction  of  gold 
and  the  remainder  of  the  silver. 


128  EXTRACTION    OF    SILVER. 


Kustel  &  Hofmann  Process. 

74.  Auriferous  silver  ores  are  roasted,  as  de- 
scribed under  §  35.  They  are  then  subjected  to  a 
process  differing  from  the  preceding  ones  in  ob- 
taining separately  the  copper,  gold  and  silver. 
After  roasting,  the  ore  must  be  moistened  on  the 
floor,  by  conveying  water  through  a  spout,  and 
mixed  with  shovels,  so  as  to  get  it  uniformly  moist, 
but  not  so  wet  as  to  interfere  with  sifting,  which, 
however,  is  not  always  necessary.  The  ore  is  then 
put  into  tubs  or  boxes,  as  represented  by  Fig.  10 
or  11,  §  61,  but  provided  with  covers,  which  can 
be  easily  screwed  air-tight  on  the  rim  of  the  box, 
having  india  rubber  between  the  box  and  the 
covers.  The  ore  should  not  reach  the  brim  of  the 
box,  but  leave  a  space  of  four  or  five  inches  at 
least  above  the  ore,  as  a  chamber  for  surplus  chlo- 
rine. When  the  box  has  been  filled,  the  cover  is 
screwed  or  otherwise  fastened  on  the  box,  and  the 
chlorine  gas  admitted  (§  11). 

The  chlorine  gas  is  generated  in  a  leaden  gen- 
erator of  the  construction  shown  in  Fig.  15,  which 
represents  a  cross  section.  It  is  a  circular  tub, 
with  an  outer  ring,  a,  six  inches  deep,  for  the  re- 
ception of  the  cover,  b.  There  is  a  similar  ring,  c, 
on  the  top  of  the  cover,  which  receives  the  collar 
fastened  to  the  leaden  stirrer,  d.  There  is  also  a 
short  leaden  pipe,  e,  bent  in  the  shape  of  an  s, 
through  which  the  sulphuric  acid  is  introduced. 


EXTRACTION    OF    SILVER. 


129 


Another  lead  pipe,  /,  conveys  the  chlorine  out  of 
the  generator.     The  vessel  is  uncovered,  and  for  a 

charge  of  three  tons  of 
roasted  ore  the  follow- 
ing ingredients  are 
introduced  :  Thirty 
pounds  of  peroxide  of 
manganese  (pulver- 
ized); thirty  to  forty 
pounds  of  common 
salt,  according  to  qual- 
ity ;  seventy-five  pounds 
of  sulphuric  acid,  of 
sixty-six  degrees  ;  and 
forty-five  pounds  of 
water.  Salt,  manga- 
nese and  water  are  in- 
troduced first,  and  the 
generator  covered.  The 
two  rings,  a  and  c,  are 
filled  with  water,  in 
order  to  close  the  gen- 
erator air  tight.  The 
sulphuric  acid  is  now 
charged  through  the 
funnel,  e,  in  small  amounts  ;  twenty  to  twenty- 
five  pounds  are  sufficient  to  evolve  the  chlo- 
rine and  the  required  heat.  When  the  evo- 
lution of  chlorine  becomes  weaker,  twenty  pounds 
more  of  acid  are  administered,  and  after  some  time 
the  rest  of  the  seventy-five  pounds.  It  will  be 
6* 


130  EXTRACTION    OF    SILVER. 

necessary  now  to  build  a  fire  beneath  the  gener- 
ator, which  is  placed  on  tiles  that  the  heat  may 
not  injure  the  leaden  bottom,  which  is  made  of 
sixteen-pound  sheet  lead,  while  the  sides  are  of 
eight-pound. 

The  chlorine  is  not  led  directly  to  the  ore,  but 
through  a  purifying  apparatus,  as  represented  in 
Fig.  16.  An  ordinary  wash  basin  or  a  similar  ves- 
sel receives  the  lead  pipes  (three-quarter  inch). 
One  of  these,  b,  conveys  the  chlorine  from  the  gen- 
erator ;  the  other,  c,  leads  the  gas  to  the  ore-box. 
Both  ends  are  covered  with  a  bottle,  a,  the  bottom 
of  which  is  cut  off  (by  meaus  of  a  string).  Clean 
water  is  poured  in  the  basin  so  as  to  cover  the  end 
of  the  pipe,  6,  about  one  inch  high.  The -gas  is 
forced  to  pass  through  the  water,  by  which  the 
muriatic  acid  is  taken  up.  Both  lead  pipes  can  be 
provided  with  rubber  hose  for  connections  with 
generator  and  ore-box.  The  passing  of  the  chlo- 
rine through  the  water  shows  distinctly  the  rapidity 
of  the  evolution  of  the  gas  in  the  generator,  and 
indicates  when  there  is  need  of  more  acid  or  of  the 
application  of  heat.  The  water  in  the  basin  ab- 
sorbs more  chlorine  when  cold — about  2J  per  cent. 
of  its  volume — before  it  is  saturated.  A  continual 
stream  of  water  is  therefore  improper. 

The  purified  chlorine  is  now  conducted  through 
the  pipe,  e,  of  Fig.  10  or  11,  §  61,  to  which  the 
rubber  hose  or  lead  pipe  from  the  purifying  appa- 
ratus (Fig.  16)  is  attached.  The  chlorine  goes 
through  the  whole  mass  of  the  ore,  driving  out  the 


EXTRACTION    OF    SILVEK.  131 

lighter  air  through  a  hole  in  the  cover  till  the  gas 
itself  comes  out  at  the  same  hole.  A  glass  rod  is 
dipped  from  time  to  time  into  ammonia,  and  held 
before  the  hole.  As  soon  as  white  fumes  appear 
on  the  rod,  it  is  proof  that  the  box  is  filled  with 
chlorine.  The  hole  is  now  closed  and  the  fire  be- 
low the  generator  removed.  In  this  condition  the 
ore  remains  for  twelve  or  fifteen  hours.  The  whole 
arrangement  must  be  examined  with  ammonia,  to 
see  that  there  is  no  loss  of  chlorine.  According  to 
the  amount  and  quality  of  the  gold,  it  may  be  neces- 
sary to  allow  the  chlorine  to  operate  for  eighteen 
to  twenty  hours. 

Lixiviation.  It  is  generally  the  case,  and  ought 
to  be  so,  that  only  a  part  of  the  chlorine  is  con- 
sumed, while  the  rest  is  unchanged.  For  the  sake 
of  economy  and  for  sanitary  considerations,  the  free 
escape  of  the  surplus  gas  should  not  be  allowed  ; 
but  this  should  be  utilized  for  the  same  purpose — 
that  is,  for  chlorination.  This  gas  is  easily  trans- 
ferred to  another  box  prepared  with  moistened  ore. 
Mr.  O.  Hofmann  inserted  a  rubber  pipe  into  the 
hole  of  the  cover  of  the  ore-box,  and  joined  the 
other  end  with  a  pipe,  e,  of  another  box,  as  in  Fig. 
10.  Through  the  pipe,  e,  of  the  already  chlori- 
dized  ore,  he  admitted  the  water,  which,  entering 
the  box,  displaced  the  chlorine  and  forced  it  into 
the  other  vat.  The  water,^  however,  dissolves  a 
part  of  the  chlorine,  and  for  this  reason  it  is  better 
to  convey  the  surplus  gas  over  by  suction,  which  is 
easily  effected  by  an  air-tight  tub  in  which  a 


132  EXTRACTION    OF    SILVER. 

vacuum  is  created  by  the  discharge  of  water.  This 
way  is  the  more  advisable,  as  a  delay,  by  which  the 
leaching  water  is  longer  in  the  box,  is  injurious  to 
the  gold.  When  the  gas  is  removed,  hot  water  is 
introduced,  and  the  leach,  containing  gold,  cop- 
per, etc.,  led  into  the  precipitating  box.  An  ad- 
dition of  dissolved  sulphate  of  iron  will  precipitate 
the  gold  in  metallic  condition.  The  sulphate  of 
iron  is  either  procured  as  an  article  of  commerce, 
or  prepared  by  throwing  old  iron  into  diluted  sul- 
phuric acid. 

After  several  hours  (from  eight  to  twelve)  the 
clear  solution  is  drawn  off  from  the  precipitated 
gold,  and  if  the  copper  is  to  be  extracted,  conveyed 
to  other  vats  and  treated  as  described  in  §  67.  The 
liquid,  coming  out  through  the  filter  of  Fig.  10, 
must  be  examined  in  the  beginning  with  a  clear  so- 
lution of  sulphate  of  iron,  and  as  long  as  a  dark 
color  of  precipitated  gold  is  perceived,  the  leach  is 
allowed  to  run  into  the  gold  precipitating  tub  ;  if 
there  is  no  precipitate  observed,  the  leach  is  di- 
rected into-  the  copper  vats.  The  liquid  is  now  ex- 
amined with  sulphide  of  sodium  or  of  calcium,  and 
proceeded  with  exactly  as  shown  in  §  61,  "  Second 
Leaching." 

Augustin   Process. 

75.  This  process  is  not  in  use  at  present  for 
silver  ores,  but  for  products  of  smelting.  By  this 
method  the  chloride  of  silver,  which  is  formed  by 


EXTRACTION    OF    SILVER.  133 

way  of  roasting  (§  30-31),  is  dissolved  in  a  hot  so- 
lution of  salt,  and  precipitated  by  metallic  copper. 
One  part  of  chloride  of  silver  requires  sixty-eight 
parts  of  salt,  to  be  dissolved. 

Extraction  of  the  Silver  from  Copper  Matt  and 
Black  Copper.  The  principal  aim  with  these  mate- 
rials is  the  oxidation  of  the  copper  as  perfectly  as 
possible,  and  then  the  chlorination  of  the  silver. 
There  are  wooden  leaching  tubs  of  a  small  size — 
two  feet  eight  inches  in  diameter,  and  nearly  four 
feet  high — fixed  on  wheels  and  arranged  in  one 
row.  Into  these  tubs,  which  have  false  bottoms, 
the  roasted  stuff  is  introduced — about  800  pounds 
in  each.  Ores  containing  different  kinds  of  earths 
cannot  be  lixiviated  at  a  depth  of  over  three  feet; 
the  metal  oxides,  however,  allow  the  water  to  pass 
freely.  This  is  also  the  case  with  roasted,  concen- 
trated, or  pure  sulphurets.  Hot  solution  of  salt  is 
now  allowed  to  flow  through  a  trough  in  each  tub. 
The  salt  penetrates  the  powder,  dissolves  the  chlo- 
ride of  silver,  and  carries  it  through  the  filter  at 
the  bottom  of  the  tubs,  flows  off  to  a  reservoir,  and 
from  here,  after  the  particles  which  may  escape 
through  the  filter  have  settled,  into  a  series  of  ves- 
sels one  above  the  other.  These  are  provided 
with  double  bottoms.  The  two  uppermost  rows 
contain  cement  copper,  six  inches  deep  ;  the  low- 
est, metallic  iron. 

The  fluid  deposits  its  silver  principally  in  the 
first  tub,  dissolving  at  the  same  time  an  equivalent 
amount  of  copper.  Some  silver  which  escapes  pre- 


134  EXTRACTION    OF    SILVER. 

cipitation  falls  with  the  cupreous  fluid  into  the  next 
tub  below,  where  the  rest  of  the  silver  is  taken  up 
by  the  copper.  In  the  third  vessel  the  copper  is 
precipitated  by  the  iron.  The  brine,  freed  from 
silver  and  copper,  is  pumped  up  into  the  reservoir, 
heated  and  used  again.  The  cement  copper  ob- 
tained in  the  last  tub  is  placed  back  in  the  upper . 
two.  The  brine  circulates  in  the  tubs  until  a 
bright  copper  plate  is  not  coated  with  silver  when 
held  in  the  fluid  from  the  leaching  tubs.  The 
residue,  which  is  mostly  copper-oxide,  is  removed, 
and  an  .average  sample  taken  and  assayed.  If  it 
should  contain  over  eight  ounces  per  ton,  it  must 
be  roasted  over  and  again  lixiviated. 

The  precipitated  silver  is  taken  out  once  a  week 
and  treated  with  muriatic  acid  for  the  purpose  of 
dissolving  the  copper  particles  which  remained 
with  the  silver.  After  this  it  is  washed  with  water 
till  all  traces  of  the  acid  disappear,  then  pressed 
into  balls,  dried  and  melted. 

Ziervogel  Process. 

76.  Like  the  preceding,  Ziervogel's  extraction 
of  silver  is  not  applied  to  silver  ores,  but  only  to 
copper  matt.  The  roasting  (§  43)  is  very  delicate, 
and  it  is  more  difficult  to  obtain  a  satisfactory  re- 
sult with  silver  ores  than  by  a  chloriclizing  roast- 
ing. The  silver  in  this  process  is  converted  into  a 
sulphate,  which  is  soluble  in  water,  thus  dispens- 
ing with  the  expensive  salt  brine.  The  pulverized 


OF  THB 

TJNIVERSr 


EXTRACTION    OF    SILVER. 


and  properly  roasted  copper  matt*  is  charged  into 
laaching  tubs,  500  pounds  in  each,  and  hot  water 
admitted.  As  soon  as  the  water  begins  to  flow  out, 
the  hot  water  is  made  a  little  acid  by  admixture  of 
some  sulphuric  acid.  The  lixiviation  continues 
until  a  sample  of  the  fluid  remains  clear  if  a  so- 
lution of  salt  is  added.  The  silver-holding  brine 
is  conveyed  into  a  large  reservoir,  thirty  feet  long, 
where  it  clears  of  impurities,  which  accidentally 
come  out  of  the  leaching  tubs,  and  falls  from  this 
Reservoir  through  a  series  of  cocks  into  the  precipi- 
tating tubs.  On  the  false  bottom  is  a  layer  of  ce- 
ment copper,  and  upon  this  fifteen  to  twenty  cop- 
per bars  of  250  pounds  weight.  Each  is  fourteen 
inches  long,  five  inches  wide  and  one  inch  thick. 
The  liquid  loses  most  of  its  silver  in  these  tubs, 
and  flows  then  through  a  trough  fifteen  inches 
wide,  lined  with  sheet  lead  and  having  a  layer  of 
copper  pieces  on  the'  bottom,  into  five  vats  filled 
with  copper,  where  the  balance  of  the  silver  is  de- 
posited. 

The  desilverized  brine  comes  now  into  a  reservoir, 
whence  it  is  pumped  up  into  a  large  leaden  pan 
and  heated  again  by  means  of  steam.  Above  this 
pan  is  a  leaden  vessel,  out  of  which  about  thirty 
drops  of  somewhat  diluted  sulphuric  acid  drop  into 
the  liquid  every  minute.  The  acid  prevents  the 
separation  of  basic  salts.  The  silver  is  taken  out 
of  the  precipitating  tubs  every  day.  With  it  occur 
some  copper  and  gypsum.  The  larger  particles  of 
copper  are  separated  by  washing,  exposed  for  six 


136  EXTRACTION    OF    SILVER. 

or  seven  days  to  leaching  with  diluted  sulphuric 
acid,  and  finally  washed  with  hot  water.  The  sil- 
ver is  from  860  to  870  fine.  After  drying,  it  is  re- 
fined in  a  reverberatory  furnace. 

Once  a  year  the  brine  is  brought  into  contact 
with  iron,  in  order  to  precipitate  the  copper.  The 
purer  part  of  the  cement  copper  is  used  for  the 
silver  precipitation,  and  the  finer  part  is  delivered 
for  smelting. 

The  Leaching  Process.*  „ 

77.  Under  this  name  is  understood  a  prepara- 
tion of  the  ore  applicable  for  the  pan  amalgamation. 
Its  description,  therefore,  does  not  belong  here 
strictly,  but  the  leaching  itself  has  so  close  a  con- 
nection with  the  preceding  manipulations  that  this 
part  alone  may  be  described  without  mentioning 
the  further  treatment  by  amalgamation. 

It  is  a  known  fact  that,  in  treating  refractory  ore  in 
pans  by  amalgamation,  of  course  by  way  of  roast- 
ing, some  very  annoying  things  are  encountered, 
and  amongst  them  principally,  the  great  loss  of 
quicksilver,  amounting  sometimes  up  to  ten  or 
twelve  pounds  per  ton  of  ore  ;  the  rapid  destruction 
of  pans,  which  compelled  many  mills  to  use 
wooden  sides  fixed  to  the  iron-pan  bottom,  a  meas- 
ure which  saves  the  pans  at  the  expense  of  quick- 
silver; and  the  very  base  bullion  which  results  from 

*The  Leaching  Process  is  patented,  as  an  application  for  pan  and 
barrel  amalgamation,  by  G.  Kustel. 


EXTRACTION    OF    SILVER.  137 

such  a  treatment.  In  some  instances  it  happens 
that  a  great  deal,  sometimes  over  fifty  per  cent. ,  of 
iron  goes  into  the  amalgam,  rendering  the  contin- 
uation of  the  amalgamation  impossible.  The  re- 
sult of  the  amalgamation  of  base  metals  is  always 
a  certain  loss  of  silver,  which  would  have  amalga- 
mated if  the  base  metals  were  out  of  the  way.  It 
happened  very  often  in  Nevada  that  $90  to  $100-ore 
was  purchased  for  the  purpose  of  amalgamating  it 
in  pans;  but  a  few  tons  proved  that  amalgamation 
had  to  be  given  up.  Such  ore  is  now  considered 
suitable  only  for  smelting. 

At  a  very  trifling  expense  all  these  difficulties  can 
be  avoided  and  the  amalgamation  turned  into  a  per- 
fect success  ;  for  instance,  the  amalgamation  of  the 
silver  ores  at  Flint,  Idaho,  (§32)  turned  out  such 
base  amalgam  that  further  working  proved  to  be 
ruinous.  The  introduction  of  the  leaching  pro- 
cess, however,  resulted  in  a  most  favorable  amal- 
gamation. It  is  only  to  be  regretted  that  after 
working  several  hundred  tons,  the  mine  refused  to 
provide  the  mill  with  ore,  perhaps  on  account  of 
not  having  been  sufficiently  opened.  The  leaching 
for  the  pan  amalgamation  is  most  important  and 
at  the  same  time  cheap  ;  all  the  expense  is  reduced 
to  that  of  obtaining  hot  water.  This  process  is 
not  only  important  for  silver  ores  containing  base 
metals,  but  also  for  gold  ores  which  by  their  na- 
ture require  roasting.  This  refers  principally  to 
auriferous  copper  ores,  as  the  amalgamation  of  gold 
is  very  much  obstructed  by  the  presence  of  copper 
salts. 


138  EXTRACTION    OF     GOLD. 

It  is  a  matter  of  surprise  how  so  simple  a  remedy 
could  have  been  overlooked  while  fighting  with  the 
obstructions,  caused  by  rebellious  ores,  during  the 
amalgamation.  If  there  is  soluble  chloride  of  sil- 
ver in  the  roasted  ore,  and  besides  this,  soluble 
chlorides  of  copper,  lead,  antimony  and  zinc,  it  is 
a  matter  of  course  that  all  will  be  decomposed  and 
amalgamated.  All  take  part  in  consuming  and 
parting  the  quicksilver,  and  in  destroying  the  pan, 
hindering  at  the  same  time  the  easy  amalgamation 
of  the  silver  and  gold.  Why,  then,  not  put  all 
these  obstructive  metals  out  of  the  way  and  give  the 
silver  a  better  chance  to  amalgamate  ?  The  base 
metal  chlorides  are  soluble  in  water,  the  chloride 
of  silver  is  not.  It  is  therefore  a  most  simple  ma- 
nipulation to  dissolve  those  salts  in  water  and  to 
remove  them  from  the  ore  before  amalgamation,  by 
the  leaching  process.  As  soon  as  this  is  done  the 
ore  is  divested  of  its  rebellious  nature  and  it  be- 
haves in  pans  like  the  best  ore. 

The  process  of  leaching  is  described  §  61  a. 


IV.    EXTRACTION  OF  GOLD. 

78.  The  extraction  of  gold  without  the  use  of 
quicksilver  is  limited  mostly  to  those  ores  in  which 
the  gold  is  not  free  in  a  metallic  condition,  but 
combined  with  sulphur  or  arsenic  in  the  respective 
pyrites. 


EXTRACTION    OF    GOLD.  139 

There  is  only  one  body  with  which  the  gold  must 
be  combined  before  subjected  to  further  treatment, 
and  this  body  is  chlorine.  The  chlorination  can  be 
effected  either  during  roasting,  §  38,  or  after  roast- 
ing, by  contact  with  chlorine  gas,  §  74,  79,  or  finally 
by  contact  with  chlorinated  water,  §  73.  The  usual 
way  of  chloridizing  the  gold  is  that  by  introducing 
the  chlorine  gas  into  the  roasted  ore,  when  cold. 
This  mode  is  described  fully  in  Kustel's  work  on 
"  Concentration  and  Chlorination."  No  improve- 
ment of  importance  has  been  since  introduced  in 
this  process. 

The  Chlorination  Process.  (Plattner's. ) 


79.  This  process  is  based  on  the  property  of 
metallic  gold  of  being  changed  into  a  soluble 
chloride  of  gold  when  in  contact  witji  chlorine  gas. 
The  chloride  of  gold  can  be  dissolved  in  water, 
separated  from  the  ore  by  lixiviation,  and  then 
precipitated  in  a  metallic  condition  by  a  solution 
of  sulphate  of  iron.  There  are  several  establish- 
ments in  California,  principally  in  Grass  Valley,  * 
where  auriferous  pyrites  are  treated  by  chlorina- 
tion on  a  large  scale.  By  way  of  chlorination,  if 
properly  executed,  90  to  95  per  cent,  of  the  fire 
assay  can  be  extracted. 


*  The  first  idea  of  trying  this  process  on  sulphurets  in  California 
came  from  Mr.  Ch.  Von  Beseler,  who  experimented  on  it  with  Mr.  Deet- 
ken  in  1858.  Since  then  Mr.  Deetken  has  been  engaged  in  the  process, 
up  to  the  present  day,  superintending  chlorination  works  in  Grass  Val- 
ley. 


140 


EXTRACTION    OF    GOLD. 


80.  In  order  to  be  sure  of  a  result  on  a  large 
scale,  it  is  an  easy  matter  to  make  an  experiment 
with  twenty  or  thirty  pounds  of  sulphurets  or  ore 
in  the  following  way  :  The  named  quantity  must 
be  roasted  first,  and  it  is  the  most  difficult  task, 
requiring  either  a  small  furnace  or  a  great  deal  of 
patience,  especially  when  small  charges  are  treated 
on  a  large  piece  of  sheet  iron,  having  a  charcoal 
fire  beneath.  In  either  case  the  sulphur  must  be 
driven  out  perfectly,  so  that  when  in  a  glowing 
condition,  no  smell  of  sulphurous  acid  can  be  ob- 
served. When  finished  a  scruple  is  taken  for  an 
assay,  and  the  roasted  stuff  moistened  with  water, 
after  the  weight  of  the  whole  has  been  noted. 

Fig.  17. 


A  common  water  bucket  is  then  prepared  to  re- 
ceive the  moistened  ore,  which  must  not  be  too  wet, 
but  only  moist  enough  to  allow  its  being  sifted. 
On  the  bottom  of  the  bucket,  a,  Fig.  17,  some  clean 
rock  or  broken  glass  is  placed  about  two  inches 
deep,  and  covered  with  a  piece  of  moistened  can- 
vas. A  short  glass  pipe,  c,  two-eighths  of  an  inch 
in  diameter,  is  inserted  close  above  the  bottom. 


EXTRACTION    OF    GOLD.  141 

The  ore,  d,  is  then  introduced,  filling  up  two- 
thirds  or  less  of  the  space  as  loosely  as  possible, 
and  covered  with  a  wooden  or  iron  cover  and 
pasted  all  around  with  dough.  The  cover  is  pro- 
vided with  a  short  glass  tube,  like  c,  to  which  an 
india  rubber  tube,-/",  for  carrying  the  gas  out  of  the 
room  is  attached.  Both  glass  tubes,  c  and*/",  must 
be  likewise  secured  with  dough. 

The  chlorine  gas  is  generated  in  a  glass  vessel, 
A*.  There  are  two  corks  in  it,  each  having  a 
glass  tube,  as  represented  in  the  drawing.  The 
cork,  I,  is  removed  and  the  vessel  charged  with 
3  ounces  of  peroxide  of  manganese,  4  ounces  of 
common  salt,  and  4J  ounces  of  water — all  of  which 
are  well  mixed.  The  cork  is  inserted  again  and 
well  secured  with  dough.  Another  vessel,  B,  pro- 
vided with  two  necks,  contains  water  as  indicated 
by  (7;  the  glass  tube,  h,  dips  about  one-half  inch 
into  the  water.  The  corks  are  made  air  tight  like 
the  others  in  A.  The  whole  apparatus  is  now  joined 
together  by  rubber  pipe,  n  and  o,  fitting  tightly  to 
the  glass  tubes.  Having  all  thus  prepared,  7-J 
ounces  of  sulphuric  acid  are  poured  through  the 
safety-tube,  m,  but  only  in  small  portions  and  at 
intervals.  When  the  bubbling  of  the  water  at  g,  in 
the  vessel  B,  is  not  lively  enough,  some  more  acid 
is  introduced,  and  finally  the  temperature  raised 
by  an  alcohol  lamp.  If  all  the  joints  have  been 
luted  carefully  with  dough,  not  the  slightest  incon- 

*  All  materials  necessary  for  such  an  apparatus  can  be  procured  from 
John  Taylor,  on  Washington  street,  San  Francisco. 


142  EXTRACTION    OF    GOLD. 

venience  will  be  met  with.  The  chlorine  gas  from 
the  generator,  A,  is  forced  through  the  water  in  JB, 
by  this  means  washed  from  muriatic  acid.  Through 
the  pipe,  o,  it  enters  the  bucket  and  ascends  slowly 
till  it  reaches  the  cover,  escaping  then  through  the 
rubber  pipe,  F,  where  it  must  be  examined  from 
time  to*time  by  dipping  a  glass  rod  into  ammonia 
and  holding  it  to  the  end  of  the  pipe,  x,  which 
leads  out  of  the  room.  In  contact  with  chlorine 
the  ammonia  evolves  white  fumes,  and  chlorine  can 
be  detected  by  these  means  wherever  it  may  escape. 
The  gas  is  allowed  to  pass  through  the  bucket  as 
long  as  chlorine  is  created.  In  this  condition,  by 
stopping  up  the  pipe,  x,  if  no  more  chlorine  is 
evolved  the  apparatus  may  stand  until  the  next 
day.  The  cover  is  then  removed,  the  pipe,  o,  taken 
off,  a  clean  glass  or  porcelain  vessel,  as  indicated 
by  z,  placed  below  c,  and  warm  water  carefully 
poured  over  the  ore  till  the  bucket  appears  to  be 
full.  The  solution  which  comes  out  at  c,  must  be 
examined  at  times  in  a  small  tumbler  with  a  few 
drops  of  a  solution  of  sulphate  of  iron.  If  the 
clear  solution  remains  unchanged,  without  becom- 
ing darker,  the  lixiviation  is  finished. 

To  the  solution  in  the  vessel,  z,  a  few  drops  of 
muriatic  acid  and  then  sulphate  of  iron,  or  green 
vitriol,  (dissolved)  is  added  and  stirred  with  a  glass 
rod.  The  whole  is  allowed  to  stand  till  all  the 
gold  is  precipitated  and  the  liquid  is  perfectly 
clear.  This  is  drawn  off  by  means  of  a  syphon, 
for  which  the  rubber  pipe,  x,  can  be  used.  The 


EXTRACTION    OF    GOLD.  143 

remaining  fluid  and  the  precipitated  gold  is  gath- 
ered on  a  filter,  washed  with  warm  water  and  dried 
with  the  filter  in  a  porcelain  cup,  above  an  alcohol 
lamp.  The  filter  is  burned  either  free  or  under  a 
muffle,  care  being  taken  not  to  lose  a  particle  of  the 
filter  ashes  ;  mixed  with  some  lead  it  is  then  cu- 
pelled and  the  gold  button  weighed.  A  compari- 
son with  the  assay  shows  to  what  percentage  the 
chlorination  has  proceeded. 

Chlorination  of  Sulphurets  and  Arseniu- 
rets. 

8 1 .  All  the  proceedings  of  the  chlorination  have 
been  treated  already  in  describing  the  process  of 
silver  to  which  we  will  here  refer.  The  first  oper- 
ation is  an  oxidizing  roasting,  as  explained  in  §  44. 
When  roasted,  the  ore  is  moistened,  charged  into 
chlorinating  vats — which  are  preferable  to  boxes  for 
gold — and  chloridized  according  to  §  74.  After  sev- 
eral chlorinations  have  been  performed  and  the  gold 
has  accumulated  in  the  precipitating  vat,  the  inside 
of  which  should  be  varnished  with  asphaltum  var- 
nish, the  gold  is  taken  out  by  means  of  a  scoop, 
put  into  a  clean  porcelain  dish  or  enameled  vessel, 
filtered,  washed  first  with  diluted  nitric  acid  and 
then  with  hot  water,  dried  and  melted  with  the  ad- 
dition of  some  borax. 

Other  methods  of  extracting  gold  without  mer- 
cury are  mentioned  in  §  38  and  §  73. 


TABLE  OF  CONTENTS. 


I.     INTRODUCTION. 

PAGE. 

Classification  of  Ores 5 

Important  Silver  Ores 5 

Difference  between  Beal  Silver  Ores  and  Argentiferous 

Ores 7 

Important  Combinations 8 

Means  of  Desulphurization 8 

Eesult  of  Desulphurization 11 

Means  of  Reduction   , , 12 

Desulphurization  of  Ores  not  Efficient 13 

What  a  Chloride  is,  and  how  Chlorination  is  effected. ...  14 

Means  of  Separating  the  Metals  from  Chlorine 16 

II.     BOASTING  OF  ORES. 

A.     Chloridizing  Boasting 22 

Necessary  amounts  of  Sulphurets 26 

Amount  of  Salt  used 28 

Permanent 'stirring  not  essential 31 

Signs  of  a  good  Chloridizing  Boasting 32 

Means  of  destroying  Base  Metal  Chlorides 35 

Steam  decomposes  Base  Metal  Chlorides 37 

Application  of  Steam  in  Boasting 37 

Lead  has  a  bad  influence 38 

Difference  in  Boasting  Processes 39 

Examples  of  Boasting  Processes 39 

In  what  condition  the  Metals  are  after  Boasting 41 

Charges  in  Boasting 67 


TABLE  OF  CONTENTS.  Cxlv 

PAGE. 

B.     Oxidizing  Roasting 67 

Chemical  Changes  in  Roasting 68 

What  Process  Requires  Oxidizing  Roasting 69 

Roasting  Furnaces 76 

Furnaces  Managed  by  Handwork 80 

Reverberatory  Furnaces 80 

Single  Roasting  Furnace 80 

Double  Roasting  Furnace " 84 

Long  Roasting  Furnace 85 

Muffle  Furnace 89 

Furnaces  with  Mechanical  Apparatus 90 

Revolving  Hearth  Furnace 91 

Ernst's  Rotary  Furnace 93 

Parke's  Furnace 95 

Bruckner's  Furnace 96 

O'Hara's  Chain  Furnace 97 

Stedefeldt's  Furnace 99 

Chimneys  and  Flues 102 

III.     EXTRACTION  OF  SILVER  BY  LIXIVIATION. 

Solving  Process 104 

Extraction  of  Silver 105 

Precipitation  of  the  Silver Ill 

Treatment  of  Precipitated  Silver 113 

Precipitation  of  Copper 115 

Quality  of  Ores  Fit  for  the  Solving  Process 121 

Sulphide  of  Calcium 123 

Hyposulphite  of  Lime 125 

Patera  Process 126 

Kiss  Process 126 

Patera  and  Roszner  Process 127 

Kustel  and  Hofmann  Process 128 

Augustin  Process 132 

Ziervogel  Process 134 

The  Leaching  Process 136 

IV.     EXTRACTION  OF  GOLD. 

The  Chlorination  Process  (Plattner's).. .        139 

Chlorination  of  Sulphurets  a.jjj^zf^iRi^pps^^ 143 

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